MySQL数据库练习【二】

一、MySQL基本语法

-- 创建数据库时指定编码方式
create database students character set utf8;
-- 查看数据库
show databases;
-- 查看已经创建的数据库信息
show create database students;
-- 修改数据库的编码
alter database students default character set utf8;
-- 删除数据库
drop database students;

-- 查看数据表
show create table student;
-- 查看表结构
desc student;
-- 修改创建好的表名 将原来的表名改为stu
alter table student rename stu;
-- 修改表中字段 将原来的字段名name改为sname字段名
alter table stu change name sname char(10);
-- 添加字段 在stu表中添加一个age整形字段
alter table student add age int;
-- 删除字段 刚刚创建的age整形字段
alter table student drop age;
-- 删除表
drop table if exists student;

二、建库建表-数据准备

create database school;
use school;

--1.学生表
CREATE TABLE `student` (
  `sid` varchar(10) NOT NULL COMMENT '学生编号',
  `sname` varchar(20) NOT NULL COMMENT '学生姓名',
  `sage` DATETIME NOT NULL COMMENT '出生年月',
  `ssex` varchar(10) NOT NULL COMMENT '学生性别'
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COMMENT='学生表';

INSERT INTO student VALUES('01' , '赵雷' , '1990-01-01' , '男'),('02' , '钱电' , '1990-12-21' , '男'),
('03' , '孙风' , '1990-05-20' , '男'),('04' , '李云' , '1990-08-06' , '男'),
('05' , '周梅' , '1991-12-01' , '女'),('06' , '吴兰' , '1992-03-01' , '女'),
('07' , '郑竹' , '1989-07-01' , '女'),('08' , '王菊' , '1990-01-20' , '女');

select * from student;
desc student;

--2.课程表
CREATE TABLE `course` (
  `cid` varchar(10) NOT NULL COMMENT '课程编号',
  `cname` varchar(10) NOT NULL COMMENT '课程名称',
  `tid` varchar(10) NOT NULL COMMENT '教师编号'
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COMMENT='课程表';

INSERT INTO course VALUES('01' , '语文' , '02'),('02' , '数学' , '01'),('03' , '英语' , '03');

select * from course;
desc course;

--3.教师表
CREATE TABLE `teacher` (
  `tid` varchar(10) NOT NULL COMMENT '教师编号',
  `tname` varchar(10) NOT NULL COMMENT '教师姓名'
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COMMENT='教师表';

INSERT INTO teacher VALUES('01' , '张三'),('02' , '李四'),('03' , '王五');

select * from teacher;
desc teacher;

--4.成绩表
CREATE TABLE `sc` (
  `sid` varchar(10) NOT NULL COMMENT '学生编号',
  `cid` varchar(10) NOT NULL COMMENT '课程编号',
  `score` DECIMAL(18,1) NOT NULL COMMENT '分数'
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COMMENT='成绩表';

INSERT INTO sc VALUES('01' , '01' , 80),('01' , '02' , 90),('01' , '03' , 99),
('02' , '01' , 70),('02' , '02' , 60),('02' , '03' , 80),('03' , '01' , 80),
('03' , '02' , 80),('03' , '03' , 80),('04' , '01' , 50),('04' , '02' , 30),
('04' , '03' , 20),('05' , '01' , 76),('05' , '02' , 87),('06' , '01' , 31),
('06' , '03' , 34),('07' , '02' , 89),('07' , '03' , 98);

select * from sc;
desc sc;

学生表student	成绩表sc	课程表course	教师表teacher
sid：学生编号	sid：学生编号	cid：课程编号	tid：教师编号
sname：学生姓名	cid：课程编号	cname：课程名称	tname：教师姓名
sage：出生年月	score：分数	tid：教师编号
ssex：学生性别

sid	sname	sage	ssex
01	赵雷	1990-01-01 00:00:00	男
02	钱电	1990-12-21 00:00:00	男
03	孙风	1990-05-20 00:00:00	男
04	李云	1990-08-06 00:00:00	男
05	周梅	1991-12-01 00:00:00	女
06	吴兰	1992-03-01 00:00:00	女
07	郑竹	1989-07-01 00:00:00	女
08	王菊	1990-01-20 00:00:00	女
09	李白	2024-02-08 00:00:00	男
10	玛利亚	2024-02-08 00:00:00	女

sid	cid	score
01	01	80.0
01	02	90.0
01	03	99.0
02	01	70.0
02	02	60.0
02	03	80.0
03	01	80.0
03	02	80.0
03	03	80.0
04	01	50.0
04	02	30.0
04	03	20.0
05	01	76.0
05	02	87.0
06	01	31.0
06	03	34.0
07	02	89.0
07	03	98.0

cid	cname	tid
01	语文	02
02	数学	01
03	英语	03

tid	tname
01	张三
02	李四
03	王五

三、MySQL经典50题-题目

4.1、查询"01"课程比"02"课程成绩高的学生的信息及课程分数
4.2、查询"01"课程比"02"课程成绩低的学生的信息及课程分数
4.3.0、查询平均成绩大于等于60分的同学的学生编号和学生姓名和平均成绩
4.3.1、总分超过200分的同学的学生编号和学生姓名和总成绩
4.4、查询平均成绩小于60分的同学的学生编号和学生姓名和平均成绩
4.5、查询所有同学的学生编号、学生姓名、选课总数、所有课程的总成绩
4.6、查询"李"姓老师的数量
4.7、查询学过"张三"老师授课的同学的信息
4.8、查询没学过"张三"老师授课的同学的信息
4.9、查询学过编号为"01"并且也学过编号为"02"的课程的同学的信息
4.10、查询学过编号为"01"但是没有学过编号为"02"的课程的同学的信息
4.11、查询没有学全所有课程的同学的信息
4.12、查询至少有一门课与学号为"01"的同学所学相同的同学的信息
4.13.0、查询和"01"号的同学学习的课程完全相同的其他同学的信息
4.13.1、MySQL中，聚合函数GROUP_CONCAT()用指定分隔符连接字符串
4.14、查询没学过"张三"老师讲授的任一门课程的学生姓名
4.15、查询两门及其以上不及格课程的同学的学号，姓名及其平均成绩
4.16、检索"01"课程分数小于60，按分数降序排列的学生信息
4.17.1、按平均成绩从高到低显示所有学生的所有课程的成绩以及平均成绩
4.17.2、CASE WHEN函数
4.17.3、CASE WHEN函数使用
4.18、查询各科成绩最高分、最低分和平均分：
4.19、按各科成绩进行排序，并显示排名
4.20、查询学生的总成绩并进行排名
4.21、查询不同老师所教不同课程平均分从高到低显示
4.22、查询所有课程的成绩第2名到第3名的学生信息及该课程成绩
4.23、统计各科成绩各分数段人数：课程编号,课程名称,[100-85],[85-70],[70-60],[0-60]及所占百分比
4.24、查询学生平均成绩及其名次
4.24.1、MySql中@符号的使用【经典】
4.25、查询各科成绩前三名的记录【经典】
4.26、查询每门课程被选修的学生数
4.27、查询出只有两门课程的全部学生的学号和姓名
4.28、查询男生、女生人数
4.29、查询名字中含有"风"字的学生信息
4.30、查询同名同性学生名单，并统计同名人数
4.31、查询1990年出生的学生名单
4.32、查询每门课程的平均成绩，结果按平均成绩降序排列，平均成绩相同时，按课程编号升序排列
4.33、查询平均成绩大于等于85的所有学生的学号、姓名和平均成绩
4.34、查询课程名称为"数学"，且分数低于60的学生姓名和分数
4.35、查询所有学生的课程及分数情况；
4.36、查询任何一门课程成绩在70分以上的姓名、课程名称和分数
4.37、查询不及格的课程
4.38、查询课程编号为01且课程成绩在80分及以上的学生的学号和姓名
4.39、求每门课程的学生人数
4.40、查询选修"张三"老师所授课程的学生中，成绩最高的学生信息及其成绩
4.41、查询不同课程成绩相同的学生的学生编号、课程编号、学生成绩
4.42、查询每门功课成绩最好的前两名【经典】
4.43、统计每门课程的学生选修人数（超过5人的课程才统计）；要求输出课程号和选修人数，查询结果按人数降序排列。
4.44、检索至少选修两门课程的学生学号
4.45、查询选修了全部课程的学生信息
4.46、查询各学生的年龄
4.47、查询本周过生日的学生
4.48、查询下周过生日的学生
4.49、查询本月过生日的学生
4.50、查询下月过生日的学生

四、MySQL经典50题-答案

4.1、查询"01"课程比"02"课程成绩高的学生的信息及课程分数

-- 学生表student、成绩表sc、课程表course、教师表teacher
-- 表结构：student、sc、course
-- 连接条件：student.sid=sc.sid
-- 先将课程为01和02的课程及对应分数筛选出来,再INNER JOIN,ON为01.sid = 02.sid,WHERE条件为01.score >02.score,结果'存'为新t3表
-- 再将student表和t3表INNER JOIN

SELECT student.*,t3.score FROM
(SELECT t1.sid,t1.score FROM 
	(SELECT sid,score FROM sc WHERE cid = "01") as t1 
	INNER JOIN (SELECT sid,score FROM sc WHERE cid = "02") as t2 
	ON t1.sid=t2.sid WHERE t1.score > t2.score 
)as t3
INNER JOIN student
ON t3.sid = student.sid;

+-----+--------+---------------------+------+-------+
| sid | sname  | sage                | ssex | score |
+-----+--------+---------------------+------+-------+
| 02  | 钱电   | 1990-12-21 00:00:00 | 男   |  70.0 |
| 04  | 李云   | 1990-08-06 00:00:00 | 男   |  50.0 |
+-----+--------+---------------------+------+-------+
2 rows in set (0.00 sec)

select
	stu.*,
	s1.score '01课程分数',
	s2.score '02课程分数'
from
	student stu,
	(select sid,score from sc where cid = '01') s1,
	(select sid,score from sc where cid = '02') s2
where s1.score > s2.score -- 前者成绩高
	and s1.sid = s2.sid 
	and s1.sid = stu.sid;

mysql> select
    -> stu.*,
    -> s1.score '01课程分数',
    -> s2.score '02课程分数'
    -> from
    -> student stu,
    -> (select sid,score from sc where cid = '01') s1,
    -> (select sid,score from sc where cid = '02') s2
    -> where s1.score > s2.score -- 前者成绩高
    -> and s1.sid = s2.sid
    -> and s1.sid = stu.sid;
+-----+--------+---------------------+------+----------------+----------------+
| SID | Sname  | Sage                | Ssex | 01课程分数     | 02课程分数     |
+-----+--------+---------------------+------+----------------+----------------+
| 02  | 钱电   | 1990-12-21 00:00:00 | 男   |           70.0 |           60.0 |
| 04  | 李云   | 1990-08-06 00:00:00 | 男   |           50.0 |           30.0 |
+-----+--------+---------------------+------+----------------+----------------+
2 rows in set (0.01 sec)

select
	stu.*,
	s1.score '01课程分数',
	s2.score '02课程分数'
from
	student stu,
	sc s1,
	sc s2
where s1.score > s2.score -- 前者成绩高
	and s1.sid = s2.sid
	and s1.cid = '01'
	and s2.cid = '02'
	and s1.sid = stu.sid;

mysql> select
    -> stu.*,
    -> s1.score '01课程分数',
    -> s2.score '02课程分数'
    -> from
    -> student stu,
    -> sc s1,
    -> sc s2
    -> where s1.score > s2.score -- 前者成绩高
    -> and s1.sid = s2.sid
    -> and s1.cid = '01'
    -> and s2.cid = '02'
    -> and s1.sid = stu.sid;
+-----+--------+---------------------+------+----------------+----------------+
| SID | Sname  | Sage                | Ssex | 01课程分数     | 02课程分数     |
+-----+--------+---------------------+------+----------------+----------------+
| 02  | 钱电   | 1990-12-21 00:00:00 | 男   |           70.0 |           60.0 |
| 04  | 李云   | 1990-08-06 00:00:00 | 男   |           50.0 |           30.0 |
+-----+--------+---------------------+------+----------------+----------------+
2 rows in set (0.00 sec)

select 
	stu.*,
	s1.score '01课程分数',
	s2.score '02课程分数'
from 
	student stu
inner join sc s1 on s1.sid = stu.sid and s1.cid = '01' -- 两个表通过学号连接，指定01
left join sc s2 on stu.sid = s2.sid and s2.cid = '02' or s2.cid = NULL -- 指定02，或者c中的c_id直接不存在
-- 为NULL的条件可以不存在，因为左连接中会直接排除c表中不存在的数据，包含NULL
where s1.score > s2.score; -- 判断条件

mysql> select 
    -> stu.*,
    -> s1.score '01课程分数',
    -> s2.score '02课程分数'
    -> from 
    -> student stu
    -> inner join sc s1 on s1.sid = stu.sid and s1.cid = '01' -- 两个表通过学号连接，指定01
    -> left join sc s2 on stu.sid = s2.sid and s2.cid = '02' or s2.cid = NULL -- 指定02，或者c中的c_id直接不存在
    -> -- 为NULL的条件可以不存在，因为左连接中会直接排除c表中不存在的数据，包含NULL
    -> where s1.score > s2.score; -- 判断条件
+-----+--------+---------------------+------+----------------+----------------+
| SID | Sname  | Sage                | Ssex | 01课程分数     | 02课程分数     |
+-----+--------+---------------------+------+----------------+----------------+
| 02  | 钱电   | 1990-12-21 00:00:00 | 男   |           70.0 |           60.0 |
| 04  | 李云   | 1990-08-06 00:00:00 | 男   |           50.0 |           30.0 |
+-----+--------+---------------------+------+----------------+----------------+
2 rows in set (0.00 sec)

4.2、查询"01"课程比"02"课程成绩低的学生的信息及课程分数

select
	stu.*,
	s1.score '01课程分数',
	s2.score '02课程分数'
from
	student stu
left join sc s1 on stu.sid = s1.sid and s1.cid = '01' or s1.cid = NULL -- 包含NULL的数据
inner join sc s2 on stu.sid = s2.sid and s2.cid = '02'
where s1.score < s2.score;

mysql> select
    -> stu.*,
    -> s1.score '01课程分数',
    -> s2.score '02课程分数'
    -> from
    -> student stu
    -> left join sc s1 on stu.sid = s1.sid and s1.cid = '01' or s1.cid = NULL -- 包含NULL的数据
    -> inner join sc s2 on stu.sid = s2.sid and s2.cid = '02'
    -> where s1.score < s2.score;
+-----+--------+---------------------+------+----------------+----------------+
| SID | Sname  | Sage                | Ssex | 01课程分数     | 02课程分数     |
+-----+--------+---------------------+------+----------------+----------------+
| 01  | 赵雷   | 1990-01-01 00:00:00 | 男   |           80.0 |           90.0 |
| 05  | 周梅   | 1991-12-01 00:00:00 | 女   |           76.0 |           87.0 |
+-----+--------+---------------------+------+----------------+----------------+
2 rows in set (0.00 sec)

4.3.0、查询平均成绩大于等于60分的同学的学生编号和学生姓名和平均成绩

select
	stu.sid '学生编号',
	stu.sname '学生姓名',
	round(avg(score),2) '平均成绩' -- round四舍五入函数
from student stu
inner join sc on stu.sid = sc.sid
group by stu.sid -- 根据sid进行分组,分组之后查询每个人的平均成绩
having avg(score) > 60; -- 先分组，后条件过滤

mysql> select
    -> stu.sid '学生编号',
    -> stu.sname '学生姓名',
    -> round(avg(score),2) '平均成绩'
    -> from student stu
    -> inner join sc on stu.sid = sc.sid
    -> group by stu.sid -- 根据sid进行分组,分组之后查询每个人的平均成绩
    -> having avg(score) > 60; -- 先分组，后条件过滤
+--------------+--------------+--------------+
| 学生编号     | 学生姓名     | 平均成绩     |
+--------------+--------------+--------------+
| 01           | 赵雷         |        89.67 |
| 02           | 钱电         |        70.00 |
| 03           | 孙风         |        80.00 |
| 05           | 周梅         |        81.50 |
| 07           | 郑竹         |        93.50 |
+--------------+--------------+--------------+
5 rows in set (0.00 sec)

4.3.1、总分超过200分的同学的学生编号和学生姓名和总成绩

select
	stu.sid '学生编号',
	stu.sname '学生姓名',
	round(sum(score),2) '总成绩'
from student stu
inner join sc on stu.sid = sc.sid
group by stu.sid
having(sum(score)) > 200;

mysql> select
    -> stu.sid '学生编号',
    -> stu.sname '学生姓名',
    -> round(sum(score),2) '总成绩'
    -> from student stu
    -> inner join sc on stu.sid = sc.sid
    -> group by stu.sid
    -> having(sum(score)) > 200;
+--------------+--------------+-----------+
| 学生编号     | 学生姓名     | 总成绩    |
+--------------+--------------+-----------+
| 01           | 赵雷         |    269.00 |
| 02           | 钱电         |    210.00 |
| 03           | 孙风         |    240.00 |
+--------------+--------------+-----------+
3 rows in set (0.00 sec)

4.4、查询平均成绩小于60分的同学的学生编号和学生姓名和平均成绩

select
	stu.sid '学生编号',
	stu.sname '学生姓名',
	round(avg(score)) '平均成绩'
from student stu
inner join sc on stu.sid = sc.sid
group by stu.sid
having avg(score) < 60;

mysql> select
    -> stu.sid '学生编号',
    -> stu.sname '学生姓名',
    -> round(avg(score)) '平均成绩'
    -> from student stu
    -> inner join sc on stu.sid = sc.sid
    -> group by stu.sid
    -> having avg(score) < 60;
+--------------+--------------+--------------+
| 学生编号     | 学生姓名     | 平均成绩     |
+--------------+--------------+--------------+
| 04           | 李云         |           33 |
| 06           | 吴兰         |           33 |
+--------------+--------------+--------------+
2 rows in set (0.00 sec)

王菊成绩表中无数据

select distinct sid from sc; -- 查询所有学号
select
	stu.sid '学生编号',
	stu.sname '学生姓名',
	0 '平均成绩'
from student stu
where stu.sid not in (select distinct sid from sc); -- 学生的学号不在表中

mysql> select
    -> stu.sid '学生编号',
    -> stu.sname '学生姓名',
    -> 0 '平均成绩'
    -> from student stu
    -> where stu.sid not in (select distinct sid from sc); -- 学生的学号不在表中
+--------------+--------------+--------------+
| 学生编号     | 学生姓名     | 平均成绩     |
+--------------+--------------+--------------+
| 08           | 王菊         |            0 |
+--------------+--------------+--------------+
1 row in set (0.00 sec)

select
	stu.sid '学生编号',
	stu.sname '学生姓名',
	round(avg(score)) '平均成绩'
from student stu
inner join sc on stu.sid = sc.sid
group by stu.sid
having avg(score) < 60
union -- 连接两张表
select
	stu.sid '学生编号',
	stu.sname '学生姓名',
	0 '平均成绩'
from student stu
where stu.sid not in (select distinct sid from sc); -- 学生的学号不在表中

mysql> select
    -> stu.sid '学生编号',
    -> stu.sname '学生姓名',
    -> round(avg(score)) '平均成绩'
    -> from student stu
    -> inner join sc on stu.sid = sc.sid
    -> group by stu.sid
    -> having avg(score) < 60
    -> union -- 连接两张表
    -> select
    -> stu.sid '学生编号',
    -> stu.sname '学生姓名',
    -> 0 '平均成绩'
    -> from student stu
    -> where stu.sid not in (select distinct sid from sc); -- 学生的学号不在表中
+--------------+--------------+--------------+
| 学生编号     | 学生姓名     | 平均成绩     |
+--------------+--------------+--------------+
| 04           | 李云         |           33 |
| 06           | 吴兰         |           33 |
| 08           | 王菊         |            0 |
+--------------+--------------+--------------+
3 rows in set (0.00 sec)

select
	stu.sid '学生编号',
	stu.sname '学生姓名',
	round(avg(ifnull(sc.score,0))) '平均成绩'
from student stu
inner join sc on stu.sid = sc.sid
group by stu.sid
having avg(sc.score) < 60;

mysql> select
    -> stu.sid '学生编号',
    -> stu.sname '学生姓名',
    -> round(avg(ifnull(sc.score,0))) '平均成绩'
    -> from student stu
    -> inner join sc on stu.sid = sc.sid
    -> group by stu.sid
    -> having avg(sc.score) < 60;
+--------------+--------------+--------------+
| 学生编号     | 学生姓名     | 平均成绩     |
+--------------+--------------+--------------+
| 04           | 李云         |           33 |
| 06           | 吴兰         |           33 |
+--------------+--------------+--------------+
2 rows in set (0.00 sec)

4.5、查询所有同学的学生编号、学生姓名、选课总数、所有课程的总成绩

select
	stu.sid '学生编号',
	stu.sname '学生姓名',
	count(sc.sid) '选课总数',
	sum(sc.score) '总成绩'
from student stu
left join sc on stu.sid = sc.sid
group by stu.sid;

mysql> select
    -> stu.sid '学生编号',
    -> stu.sname '学生姓名',
    -> count(sc.sid) '选课总数',
    -> sum(sc.score) '总成绩'
    -> from student stu
    -> left join sc on stu.sid = sc.sid
    -> group by stu.sid;
+--------------+--------------+--------------+-----------+
| 学生编号     | 学生姓名     | 选课总数     | 总成绩    |
+--------------+--------------+--------------+-----------+
| 01           | 赵雷         |            3 |     269.0 |
| 02           | 钱电         |            3 |     210.0 |
| 03           | 孙风         |            3 |     240.0 |
| 04           | 李云         |            3 |     100.0 |
| 05           | 周梅         |            2 |     163.0 |
| 06           | 吴兰         |            2 |      65.0 |
| 07           | 郑竹         |            2 |     187.0 |
| 08           | 王菊         |            0 |      NULL |
+--------------+--------------+--------------+-----------+
8 rows in set (0.00 sec)

4.6、查询"李"姓老师的数量

select
	count(t.tname)
from teacher t
where t.tname like '李%'; -- 通配符

mysql> select
    -> count(t.tname)
    -> from teacher t
    -> where t.tname like '李%'; -- 通配符
+----------------+
| count(t.tname) |
+----------------+
|              1 |
+----------------+
1 row in set (0.00 sec)

select
	t.tname,
	count(t.tname)
from teacher t
where left(t.tname,1) in ('李'); -- left函数

mysql> select
    -> t.tname,
    -> count(t.tname)
    -> from teacher t
    -> where left(t.tname,1) in ('李'); -- left函数
+--------+----------------+
| tname  | count(t.tname) |
+--------+----------------+
| 李四   |              1 |
+--------+----------------+
1 row in set (0.00 sec)

4.7、查询学过"张三"老师授课的同学的信息

-- 查询课程cid
select cid 
from teacher,course
where teacher.tid = course.tid and teacher.tname = '张三';

select
	stu.*
from student stu,sc
where  stu.sid = sc.sid
and sc.cid in (
	select cid 
	from teacher,course
	where teacher.tid = course.tid and teacher.tname = '张三');

mysql> select
    -> stu.*
    -> from student stu,sc
    -> where  stu.sid = sc.sid
    -> and sc.cid in (
    -> select cid 
    -> from teacher,course
    -> where teacher.tid = course.tid and teacher.tname = '张三');
+-----+--------+---------------------+------+
| sid | sname  | sage                | ssex |
+-----+--------+---------------------+------+
| 01  | 赵雷   | 1990-01-01 00:00:00 | 男   |
| 02  | 钱电   | 1990-12-21 00:00:00 | 男   |
| 03  | 孙风   | 1990-05-20 00:00:00 | 男   |
| 04  | 李云   | 1990-08-06 00:00:00 | 男   |
| 05  | 周梅   | 1991-12-01 00:00:00 | 女   |
| 07  | 郑竹   | 1989-07-01 00:00:00 | 女   |
+-----+--------+---------------------+------+
6 rows in set (0.00 sec)

select stu.*
from teacher t
left join course c on t.tid = c.tid -- 课程表和教师表
left join sc on c.cid = sc.cid -- 成绩表和课程表
left join student stu on sc.sid = stu.sid -- 学生表和成绩表
where t.tname = '张三';

mysql> select stu.*
    -> from teacher t
    -> left join course c on t.tid = c.tid -- 课程表和教师表
    -> left join sc on c.cid = sc.cid -- 成绩表和课程表
    -> left join student stu on sc.sid = stu.sid -- 学生表和成绩表
    -> where t.tname = '张三';
+------+--------+---------------------+------+
| sid  | sname  | sage                | ssex |
+------+--------+---------------------+------+
| 01   | 赵雷   | 1990-01-01 00:00:00 | 男   |
| 02   | 钱电   | 1990-12-21 00:00:00 | 男   |
| 03   | 孙风   | 1990-05-20 00:00:00 | 男   |
| 04   | 李云   | 1990-08-06 00:00:00 | 男   |
| 05   | 周梅   | 1991-12-01 00:00:00 | 女   |
| 07   | 郑竹   | 1989-07-01 00:00:00 | 女   |
+------+--------+---------------------+------+
6 rows in set (0.00 sec)

4.8、查询没学过"张三"老师授课的同学的信息

select distinct sc.sid
from teacher t
left join course c on t.tid = c.tid
left join sc on c.cid = sc.cid
where t.tname = '张三';

select stu.*
from student stu
where sid not in (
	select distinct sc.sid
	from teacher t
	left join course c on t.tid = c.tid
	left join sc on c.cid = sc.cid
	where t.tname = '张三');

mysql> select stu.*
    -> from student stu
    -> where sid not in (
    -> select distinct sc.sid
    -> from teacher t
    -> left join course c on t.tid = c.tid
    -> left join sc on c.cid = sc.cid
    -> where t.tname = '张三');
+-----+--------+---------------------+------+
| sid | sname  | sage                | ssex |
+-----+--------+---------------------+------+
| 06  | 吴兰   | 1992-03-01 00:00:00 | 女   |
| 08  | 王菊   | 1990-01-20 00:00:00 | 女   |
+-----+--------+---------------------+------+
2 rows in set (0.00 sec)

4.9、查询学过编号为"01"并且也学过编号为"02"的课程的同学的信息

select s1.sid
from sc s1
inner join sc s2 on s1.sid = s2.sid
where s1.cid = '01' and s2.cid = '02';

select stu.*
from student stu
where stu.sid in (
	select s1.sid
	from sc s1
	inner join sc s2 on s1.sid = s2.sid
	where s1.cid = '01' and s2.cid = '02');

mysql> select stu.*
    -> from student stu
    -> where stu.sid in (
    -> select s1.sid
    -> from sc s1
    -> inner join sc s2 on s1.sid = s2.sid
    -> where s1.cid = '01' and s2.cid = '02');
+-----+--------+---------------------+------+
| sid | sname  | sage                | ssex |
+-----+--------+---------------------+------+
| 01  | 赵雷   | 1990-01-01 00:00:00 | 男   |
| 02  | 钱电   | 1990-12-21 00:00:00 | 男   |
| 03  | 孙风   | 1990-05-20 00:00:00 | 男   |
| 04  | 李云   | 1990-08-06 00:00:00 | 男   |
| 05  | 周梅   | 1991-12-01 00:00:00 | 女   |
+-----+--------+---------------------+------+
5 rows in set (0.00 sec)

select stu.*
from student stu,sc s1,sc s2
where s1.sid = s2.sid
and stu.sid = s1.sid
and s1.cid = '01'
and s2.cid = '02';

mysql> select stu.*
    -> from student stu,sc s1,sc s2
    -> where s1.sid = s2.sid
    -> and stu.sid = s1.sid
    -> and s1.cid = '01'
    -> and s2.cid = '02';
+-----+--------+---------------------+------+
| sid | sname  | sage                | ssex |
+-----+--------+---------------------+------+
| 01  | 赵雷   | 1990-01-01 00:00:00 | 男   |
| 02  | 钱电   | 1990-12-21 00:00:00 | 男   |
| 03  | 孙风   | 1990-05-20 00:00:00 | 男   |
| 04  | 李云   | 1990-08-06 00:00:00 | 男   |
| 05  | 周梅   | 1991-12-01 00:00:00 | 女   |
+-----+--------+---------------------+------+
5 rows in set (0.00 sec)

4.10、查询学过编号为"01"但是没有学过编号为"02"的课程的同学的信息

-- 查询学过02课程的学生sid
select sid from sc  where sc.cid = '02';

select sid 
from sc 
where sc.cid = '01' -- 修过01课程的学号
and sc.sid not in (select sid from sc where sc.cid = '02'); -- 同时学号不能在修过02课程中出现

select stu.*
from student stu
where stu.sid in(
	select sid 
	from sc 
	where sc.cid = '01' 
	and sc.sid not in (select sid from sc where sc.cid = '02')
);

mysql> select stu.*
    -> from student stu
    -> where stu.sid in(
    -> select sid 
    -> from sc 
    -> where sc.cid = '01' 
    -> and sc.sid not in (select sid from sc where sc.cid = '02')
    -> );
+-----+--------+---------------------+------+
| sid | sname  | sage                | ssex |
+-----+--------+---------------------+------+
| 06  | 吴兰   | 1992-03-01 00:00:00 | 女   |
+-----+--------+---------------------+------+
1 row in set (0.00 sec)

4.11、查询没有学全所有课程的同学的信息

select stu.*
from student stu
left join sc on stu.sid = sc.sid
group by stu.sid -- 学号分组
having count(sc.cid) < (select count(*) from course); -- 筛选分组后学生的课程数<3

mysql> select stu.*
    -> from student stu
    -> left join sc on stu.sid = sc.sid
    -> group by stu.sid -- 学号分组
    -> having count(sc.cid) < (select count(*) from course); -- 筛选分组后学生的课程数<3
+-----+--------+---------------------+------+
| sid | sname  | sage                | ssex |
+-----+--------+---------------------+------+
| 05  | 周梅   | 1991-12-01 00:00:00 | 女   |
| 06  | 吴兰   | 1992-03-01 00:00:00 | 女   |
| 07  | 郑竹   | 1989-07-01 00:00:00 | 女   |
| 08  | 王菊   | 1990-01-20 00:00:00 | 女   |
+-----+--------+---------------------+------+
4 rows in set (0.00 sec)

select sc.sid
from sc
group by sc.sid
having count(sc.cid) = (select count(*) from course);

mysql> select sc.sid
    -> from sc
    -> group by sc.sid
    -> having count(sc.cid) = (select count(*) from course);
+-----+
| sid |
+-----+
| 01  |
| 02  |
| 03  |
| 04  |
+-----+
4 rows in set (0.00 sec)

select stu.*
from student stu
where stu.sid not in (
	select sc.sid
	from sc
	group by sc.sid
	having count(sc.cid) = (select count(*) from course));

mysql> select stu.*
    -> from student stu
    -> where stu.sid not in (
    -> select sc.sid
    -> from sc
    -> group by sc.sid
    -> having count(sc.cid) = (select count(*) from course));
+-----+--------+---------------------+------+
| sid | sname  | sage                | ssex |
+-----+--------+---------------------+------+
| 05  | 周梅   | 1991-12-01 00:00:00 | 女   |
| 06  | 吴兰   | 1992-03-01 00:00:00 | 女   |
| 07  | 郑竹   | 1989-07-01 00:00:00 | 女   |
| 08  | 王菊   | 1990-01-20 00:00:00 | 女   |
+-----+--------+---------------------+------+
4 rows in set (0.00 sec)

4.12、查询至少有一门课与学号为"01"的同学所学相同的同学的信息

-- 查询01号同学的所学课程cid
select sc.cid from sc where sc.sid = '01';

select stu.*
from student stu
inner join sc on stu.sid = sc.sid -- 学生表和成绩表的关联
and sc.cid in (select sc.cid from sc where sc.sid = '01') -- 对课程进行限制，只在01学生的课程内
group by stu.sid; -- 根据学号进行分组

mysql> select stu.*
    -> from student stu
    -> inner join sc on stu.sid = sc.sid -- 学生表和成绩表的关联
    -> and sc.cid in (select sc.cid from sc where sc.sid = '01') -- 对课程进行限制，只在01学生的课程内
    -> group by stu.sid; -- 根据学号进行分组
+-----+--------+---------------------+------+
| sid | sname  | sage                | ssex |
+-----+--------+---------------------+------+
| 01  | 赵雷   | 1990-01-01 00:00:00 | 男   |
| 02  | 钱电   | 1990-12-21 00:00:00 | 男   |
| 03  | 孙风   | 1990-05-20 00:00:00 | 男   |
| 04  | 李云   | 1990-08-06 00:00:00 | 男   |
| 05  | 周梅   | 1991-12-01 00:00:00 | 女   |
| 06  | 吴兰   | 1992-03-01 00:00:00 | 女   |
| 07  | 郑竹   | 1989-07-01 00:00:00 | 女   |
+-----+--------+---------------------+------+
7 rows in set (0.00 sec)

4.13.0、查询和"01"号的同学学习的课程完全相同的其他同学的信息

-- 查询01号同学学习的课程cid
select sc.cid from sc where sc.sid = '01';
-- 查询01号同学学习的课程cid总数
select count(sc.cid) from sc where sc.sid = '01';

select stu.*
from student stu
inner join sc on stu.sid = sc.sid 
	and sc.cid in (select sc.cid from sc where sc.sid = '01') -- 和01同学课程相同
	and sc.sid != '01' -- 排除自身
group by stu.sid -- 根据sid进行分组
having (select count(sc.cid) from sc where sc.sid = stu.sid) = -- 和01同学所学课程总数相同
	(select count(sc.cid) from sc where sc.sid = '01');

mysql> select stu.*
    -> from student stu
    -> inner join sc on stu.sid = sc.sid 
    -> and sc.cid in (select sc.cid from sc where sc.sid = '01') -- 和01同学课程相同
    -> and sc.sid != '01' -- 排除自身
    -> group by stu.sid -- 根据sid进行分组
    -> having (select count(sc.cid) from sc where sc.sid = stu.sid) = -- 和01同学所学课程总数相同
    -> (select count(sc.cid) from sc where sc.sid = '01');
+-----+--------+---------------------+------+
| sid | sname  | sage                | ssex |
+-----+--------+---------------------+------+
| 02  | 钱电   | 1990-12-21 00:00:00 | 男   |
| 03  | 孙风   | 1990-05-20 00:00:00 | 男   |
| 04  | 李云   | 1990-08-06 00:00:00 | 男   |
+-----+--------+---------------------+------+
3 rows in set (0.00 sec)

4.13.1、MySQL中，聚合函数GROUP_CONCAT()用指定分隔符连接字符串

-- 语法
group_concat([DISTINCT] 要连接的字段 [Order BY ASC/DESC 排序字段] [Separator '分隔符'])

select sid,group_concat(cid order by cid)
from sc
group by sid;

mysql> select sid,group_concat(cid order by cid)
    -> from sc
    -> group by sid;
+-----+--------------------------------+
| sid | group_concat(cid order by cid) |
+-----+--------------------------------+
| 01  | 01,02,03                       |
| 02  | 01,02,03                       |
| 03  | 01,02,03                       |
| 04  | 01,02,03                       |
| 05  | 01,02                          |
| 06  | 01,03                          |
| 07  | 02,03                          |
+-----+--------------------------------+
7 rows in set (0.00 sec)

select sid
from sc
group by sid
having group_concat(cid order by cid) = ( -- 找出和01号学生分组合并结果相同的学号sid
	select group_concat(cid order by cid) -- 01号学生分组合并，同时进行排序
	from sc
	group by sid
	having sid = '01')
	and sid != '01'; -- 排除01号同学

mysql> select sid
    -> from sc
    -> group by sid
    -> having group_concat(cid order by cid) = (
    -> select group_concat(cid order by cid)
    -> from sc
    -> group by sid
    -> having sid = '01')
    -> and sid != '01';
+-----+
| sid |
+-----+
| 02  |
| 03  |
| 04  |
+-----+
3 rows in set (0.00 sec)

4.14、查询没学过"张三"老师讲授的任一门课程的学生姓名

-- 查询张三老师的课程cid
select cid
from course s
inner join teacher t on s.tid = t.tid and t.tname = '张三';

-- 查询学过张三老师课程的学生sid
select sid
from sc
where cid in (
	select cid
	from course s
	inner join teacher t on s.tid = t.tid and t.tname = '张三');

select stu.*
from student stu
where stu.sid not in (
select sid from sc where cid in (
	select cid from course s inner join teacher t on s.tid = t.tid and t.tname = '张三')
);

mysql> select stu.*
    -> from student stu
    -> where stu.sid not in (
    -> select sid from sc where cid in (
    -> select cid from course s inner join teacher t on s.tid = t.tid and t.tname = '张三')
    -> );
+-----+--------+---------------------+------+
| sid | sname  | sage                | ssex |
+-----+--------+---------------------+------+
| 06  | 吴兰   | 1992-03-01 00:00:00 | 女   |
| 08  | 王菊   | 1990-01-20 00:00:00 | 女   |
+-----+--------+---------------------+------+
2 rows in set (0.00 sec)

4.15、查询两门及其以上不及格课程的同学的学号，姓名及其平均成绩

-- 查询成绩不合格同学的学号
select sid
from sc
where score < 60 -- 小于60分，不及格
group by sid
having count(*) >= 2; -- 不及格两门以上

select stu.sname,stu.sid,round(avg(score),2)
from student stu
left join sc on stu.sid = sc.sid
where stu.sid in (
	select sid
	from sc
	where score < 60
	group by sid
	having count(*) >= 2)
group by stu.sid;

mysql> select stu.sname,stu.sid,round(avg(score),2)
    -> from student stu
    -> left join sc on stu.sid = sc.sid
    -> where stu.sid in (
    -> select sid
    -> from sc
    -> where score < 60
    -> group by sid
    -> having count(*) >= 2)
    -> group by stu.sid;
+--------+-----+---------------------+
| sname  | sid | round(avg(score),2) |
+--------+-----+---------------------+
| 李云   | 04  |               33.33 |
| 吴兰   | 06  |               32.50 |
+--------+-----+---------------------+
2 rows in set (0.00 sec)

select stu.sname,sc.sid,round(avg(score),2)
from sc
inner join student stu on sc.sid = stu.sid
where sc.score < 60
group by sc.sid
having count(sc.score) >= 2;

mysql> select stu.sname,sc.sid,round(avg(score),2)
    -> from sc
    -> inner join student stu on sc.sid = stu.sid
    -> where sc.score < 60
    -> group by sc.sid
    -> having count(sc.score) >= 2;
+--------+-----+---------------------+
| sname  | sid | round(avg(score),2) |
+--------+-----+---------------------+
| 李云   | 04  |               33.33 |
| 吴兰   | 06  |               32.50 |
+--------+-----+---------------------+
2 rows in set (0.00 sec)

select stu.sid,stu.sname,round(avg(score),2)
from student stu
inner join sc on stu.sid = sc.sid and sc.score < 60 -- 内连接且分数小于60
group by stu.sid  -- 根据sid分组
having count(sc.sid) >= 2;  -- 两门课

mysql> select stu.sid,stu.sname,round(avg(score),2)
    -> from student stu
    -> inner join sc on stu.sid = sc.sid and sc.score < 60 -- 内连接且分数小于60
    -> group by stu.sid  -- 根据sid分组
    -> having count(sc.sid) >= 2;  -- 两门课
+-----+--------+---------------------+
| sid | sname  | round(avg(score),2) |
+-----+--------+---------------------+
| 04  | 李云   |               33.33 |
| 06  | 吴兰   |               32.50 |
+-----+--------+---------------------+
2 rows in set (0.00 sec)

4.16、检索"01"课程分数小于60，按分数降序排列的学生信息

select stu.*,sc.score
from student stu
left join sc on stu.sid = sc.sid
where sc.cid = '01' and sc.score < 60
order by score desc;

mysql> select stu.*,sc.score
    -> from student stu
    -> left join sc on stu.sid = sc.sid
    -> where sc.cid = '01' and sc.score < 60
    -> order by score desc;
+-----+--------+---------------------+------+-------+
| sid | sname  | sage                | ssex | score |
+-----+--------+---------------------+------+-------+
| 04  | 李云   | 1990-08-06 00:00:00 | 男   |  50.0 |
| 06  | 吴兰   | 1992-03-01 00:00:00 | 女   |  31.0 |
+-----+--------+---------------------+------+-------+
2 rows in set (0.01 sec)

4.17.1、按平均成绩从高到低显示所有学生的所有课程的成绩以及平均成绩

-- 查询每位同学平均成绩，并降序
select sc.sid,avg(score) from sc group by sc.sid order by avg(score) desc;

select
	s.sid,
	(select sname from student where student.sid = s.sid) '姓名',
	(select score from sc where sc.sid = s.sid and sc.cid = '01') '语文',
	(select score from sc where sc.sid = s.sid and sc.cid = '02') '数学',
	(select score from sc where sc.sid = s.sid and sc.cid = '03') '英语',
	round(avg(s.score),2) '平均成绩'
from sc s
group by s.sid 
order by avg(s.score) desc;

mysql> select
    -> s.sid,
    -> (select sname from student where student.sid = s.sid) '姓名',
    -> (select score from sc where sc.sid = s.sid and sc.cid = '01') '语文',
    -> (select score from sc where sc.sid = s.sid and sc.cid = '02') '数学',
    -> (select score from sc where sc.sid = s.sid and sc.cid = '03') '英语',
    -> round(avg(s.score),2) '平均成绩'
    -> from sc s
    -> group by s.sid 
    -> order by avg(s.score) desc;
+-----+--------+--------+--------+--------+--------------+
| sid | 姓名   | 语文    | 数学    | 英语    |  平均成绩     |
+-----+--------+--------+--------+--------+--------------+
| 07  | 郑竹   |   NULL |   89.0 |   98.0 |        93.50 |
| 01  | 赵雷   |   80.0 |   90.0 |   99.0 |        89.67 |
| 05  | 周梅   |   76.0 |   87.0 |   NULL |        81.50 |
| 03  | 孙风   |   80.0 |   80.0 |   80.0 |        80.00 |
| 02  | 钱电   |   70.0 |   60.0 |   80.0 |        70.00 |
| 04  | 李云   |   50.0 |   30.0 |   20.0 |        33.33 |
| 06  | 吴兰   |   31.0 |   NULL |   34.0 |        32.50 |
+-----+--------+--------+--------+--------+--------------+
7 rows in set (0.00 sec)

没有第八号学生

4.17.2、CASE WHEN函数

CASE SCORE WHEN 'A' THEN '优' ELSE '不及格' END
-- THEN后边的值与ELSE后边的值类型应一致，否则会报错
-- 使用 IF 函数进行替换
IF(SCORE = 'A', '优', '不及格')

-- CASE WHEN条件表达式函数
CASE WHEN condition THEN result
 
[WHEN...THEN...]
 
ELSE result
 
END

CASE 
	 WHEN SCORE > 90 THEN '优'
     WHEN SCORE > 70 THEN '良'
     WHEN SCORE > 60 THEN '中' 
     ELSE '不及格' END

4.17.3、CASE WHEN函数使用

select
	stu.sname,
	sc.sid,
	sum(case when sc.cid = '01' then ifnull(sc.score,0) else 0 end) '语文',
	sum(case when sc.cid = '02' then ifnull(sc.score,0) else 0 end) '数学',
	sum(case when sc.cid = '03' then ifnull(sc.score,0) else 0 end) '英语',
	round(avg(sc.score),2) '平均成绩'
from sc
left join student stu on sc.sid = stu.sid
group by sc.sid
union
select stu.sname,stu.sid,0 '语文',0 '数学',0 '英语',0 '平均成绩'
from student stu
where stu.sid not in (select distinct sid from sc)
order by 6 desc;

mysql> select
    -> stu.sname,
    -> sc.sid,
    -> sum(case when sc.cid = '01' then ifnull(sc.score,0) else 0 end) '语文',
    -> sum(case when sc.cid = '02' then ifnull(sc.score,0) else 0 end) '数学',
    -> sum(case when sc.cid = '03' then ifnull(sc.score,0) else 0 end) '英语',
    -> round(avg(sc.score),2) '平均成绩'
    -> from sc
    -> left join student stu on sc.sid = stu.sid
    -> group by sc.sid
    -> union
    -> select stu.sname,stu.sid,0 '语文',0 '数学',0 '英语',0 '平均成绩'
    -> from student stu
    -> where stu.sid not in (select distinct sid from sc)
    -> order by 6 desc;
+--------+-----+--------+--------+--------+--------------+
| sname | sid |  语文   | 数学    | 英语   | 平均成绩     |
+--------+-----+--------+--------+--------+--------------+
| 郑竹   | 07  |    0.0 |   89.0 |   98.0 |        93.50 |
| 赵雷   | 01  |   80.0 |   90.0 |   99.0 |        89.67 |
| 周梅   | 05  |   76.0 |   87.0 |    0.0 |        81.50 |
| 孙风   | 03  |   80.0 |   80.0 |   80.0 |        80.00 |
| 钱电   | 02  |   70.0 |   60.0 |   80.0 |        70.00 |
| 李云   | 04  |   50.0 |   30.0 |   20.0 |        33.33 |
| 吴兰   | 06  |   31.0 |    0.0 |   34.0 |        32.50 |
| 王菊   | 08  |    0.0 |    0.0 |    0.0 |         0.00 |
+--------+-----+--------+--------+--------+--------------+
8 rows in set (0.00 sec)

有第八号学生

select
	stu.sname,
	stu.sid,
	s1.score '语文',
	s2.score '数学',
	s3.score '英语',
	round(avg(s.score),2) '平均成绩'
from student stu
left join sc s on stu.sid = s.sid
left join sc s1 on s.sid = s1.sid and s1.cid = '01'
left join sc s2 on s.sid = s2.sid and s2.cid = '02'
left join sc s3 on s.sid = s3.sid and s3.cid = '03'
group by stu.sid
order by 6 desc;

mysql> select
    -> stu.sname,
    -> stu.sid,
    -> s1.score '语文',
    -> s2.score '数学',
    -> s3.score '英语',
    -> round(avg(s.score),2) '平均成绩'
    -> from student stu
    -> left join sc s on stu.sid = s.sid
    -> left join sc s1 on s.sid = s1.sid and s1.cid = '01'
    -> left join sc s2 on s.sid = s2.sid and s2.cid = '02'
    -> left join sc s3 on s.sid = s3.sid and s3.cid = '03'
    -> group by stu.sid
    -> order by 6 desc;
+--------+-----+--------+--------+--------+--------------+
| sname  | sid | 语文   | 数学   | 英语   | 平均成绩     |
+--------+-----+--------+--------+--------+--------------+
| 郑竹   | 07  |   NULL |   89.0 |   98.0 |        93.50 |
| 赵雷   | 01  |   80.0 |   90.0 |   99.0 |        89.67 |
| 周梅   | 05  |   76.0 |   87.0 |   NULL |        81.50 |
| 孙风   | 03  |   80.0 |   80.0 |   80.0 |        80.00 |
| 钱电   | 02  |   70.0 |   60.0 |   80.0 |        70.00 |
| 李云   | 04  |   50.0 |   30.0 |   20.0 |        33.33 |
| 吴兰   | 06  |   31.0 |   NULL |   34.0 |        32.50 |
| 王菊   | 08  |   NULL |   NULL |   NULL |         NULL |
+--------+-----+--------+--------+--------+--------------+
8 rows in set (0.00 sec)

-- 非空判断
select
	stu.sname,
	stu.sid,
	if(s1.score is NULL,0,s1.score) '语文',
	if(s2.score is NULL,0,s2.score) '数学',
	if(s3.score is NULL,0,s3.score) '英语',
	if(round(avg(s.score),2) is NULL,0,round(avg(s.score),2)) '平均成绩'
from student stu
left join sc s on stu.sid = s.sid
left join sc s1 on s.sid = s1.sid and s1.cid = '01'
left join sc s2 on s.sid = s2.sid and s2.cid = '02'
left join sc s3 on s.sid = s3.sid and s3.cid = '03'
group by stu.sid
order by 6 desc;

mysql> select
    -> stu.sname,
    -> stu.sid,
    -> if(s1.score is NULL,0,s1.score) '语文',
    -> if(s2.score is NULL,0,s2.score) '数学',
    -> if(s3.score is NULL,0,s3.score) '英语',
    -> if(round(avg(s.score),2) is NULL,0,round(avg(s.score),2)) '平均成绩'
    -> from student stu
    -> left join sc s on stu.sid = s.sid
    -> left join sc s1 on s.sid = s1.sid and s1.cid = '01'
    -> left join sc s2 on s.sid = s2.sid and s2.cid = '02'
    -> left join sc s3 on s.sid = s3.sid and s3.cid = '03'
    -> group by stu.sid
    -> order by 6 desc;
+--------+-----+--------+--------+--------+--------------+
| sname  | sid | 语文   | 数学    | 英语   | 平均成绩     |
+--------+-----+--------+--------+--------+--------------+
| 郑竹   | 07  |    0.0 |   89.0 |   98.0 |        93.50 |
| 赵雷   | 01  |   80.0 |   90.0 |   99.0 |        89.67 |
| 周梅   | 05  |   76.0 |   87.0 |    0.0 |        81.50 |
| 孙风   | 03  |   80.0 |   80.0 |   80.0 |        80.00 |
| 钱电   | 02  |   70.0 |   60.0 |   80.0 |        70.00 |
| 李云   | 04  |   50.0 |   30.0 |   20.0 |        33.33 |
| 吴兰   | 06  |   31.0 |    0.0 |   34.0 |        32.50 |
| 王菊   | 08  |    0.0 |    0.0 |    0.0 |            0 |
+--------+-----+--------+--------+--------+--------------+
8 rows in set (0.00 sec)

4.18、查询各科成绩最高分、最低分和平均分：

以如下形式显示：课程ID，课程name，最高分，最低分，平均分，及格率，中等率，优良率，优秀率
及格：>=60，中等为：70-80，优良为：80-90，优秀为：>=90

case when score then 1 else 0 end -- 总人数
case when score >= 60 then 1 else 0 end -- 及格
case when score > 70 and score <= 80 then 1 else 0 end -- 中等
case when score > 80 and score < 90 then 1 else 0 end -- 优良
case when score >= 90 then 1 else 0 end -- 优秀

select
	sc.cid 课程ID,
	c.cname 课程name,
	max(sc.score) 最高分,
	min(sc.score) 最低分,
	round(avg(sc.score),2) 平均分,
	round(100*(sum(case when score >= 60 then 1 else 0 end) / sum(case when score then 1 else 0 end)),2) 及格率,
	round(100*(sum(case when score > 70 and score <= 80 then 1 else 0 end) / sum(case when score then 1 else 0 end)),2) 中等率,
	round(100*(sum(case when score > 80 and score < 90 then 1 else 0 end) / sum(case when score then 1 else 0 end)),2) 优良率,
	round(100*(sum(case when score >= 90 then 1 else 0 end) / sum(case when score then 1 else 0 end)),2) 优秀率
from sc
left join course c on sc.cid = c.cid
group by sc.cid,c.cname;

mysql> select
    -> sc.cid 课程ID,
    -> c.cname 课程name,
    -> max(sc.score) 最高分,
    -> min(sc.score) 最低分,
    -> round(avg(sc.score),2) 平均分,
    -> round(100*(sum(case when score >= 60 then 1 else 0 end) / sum(case when score then 1 else 0 end)),2) 及格率,
    -> round(100*(sum(case when score > 70 and score <= 80 then 1 else 0 end) / sum(case when score then 1 else 0 end)),2) 中等率,
    -> round(100*(sum(case when score > 80 and score < 90 then 1 else 0 end) / sum(case when score then 1 else 0 end)),2) 优良率,
    -> round(100*(sum(case when score >= 90 then 1 else 0 end) / sum(case when score then 1 else 0 end)),2) 优秀率
    -> from sc
    -> left join course c on sc.cid = c.cid
    -> group by sc.cid,c.cname;
+----------+------------+-----------+-----------+-----------+-----------+-----------+-----------+-----------+
|  课程ID   | 课程name   |  最高分    |  最低分    |  平均分    |  及格率    |  中等率    |  优良率    |  优秀率    |
+----------+------------+-----------+-----------+-----------+-----------+-----------+-----------+-----------+
| 01       | 语文       |      80.0 |      31.0 |     64.50 |     66.67 |     50.00 |      0.00 |      0.00 |
| 02       | 数学       |      90.0 |      30.0 |     72.67 |     83.33 |     16.67 |     33.33 |     16.67 |
| 03       | 英语       |      99.0 |      20.0 |     68.50 |     66.67 |     33.33 |      0.00 |     33.33 |
+----------+------------+-----------+-----------+-----------+-----------+-----------+-----------+-----------+
3 rows in set (0.00 sec)

count(1) 替换 sum(case when score then 1 else 0 end) 总人数

select
	sc.cid 课程ID,
	c.cname 课程name,
	max(sc.score) 最高分,
	min(sc.score) 最低分,
	round(avg(sc.score),2) 平均分,
	round(100*(sum(case when score >= 60 then 1 else 0 end) / count(1)),2) 及格率,
	round(100*(sum(case when score > 70 and score <= 80 then 1 else 0 end) / count(1)),2) 中等率,
	round(100*(sum(case when score > 80 and score < 90 then 1 else 0 end) / count(1)),2) 优良率,
	round(100*(sum(case when score >= 90 then 1 else 0 end) / count(1)),2) 优秀率
from sc
left join course c on sc.cid = c.cid
group by sc.cid,c.cname;

mysql> select
    -> sc.cid 课程ID,
    -> c.cname 课程name,
    -> max(sc.score) 最高分,
    -> min(sc.score) 最低分,
    -> round(avg(sc.score),2) 平均分,
    -> round(100*(sum(case when score >= 60 then 1 else 0 end) / count(1)),2) 及格率,
    -> round(100*(sum(case when score > 70 and score <= 80 then 1 else 0 end) / count(1)),2) 中等率,
    -> round(100*(sum(case when score > 80 and score < 90 then 1 else 0 end) / count(1)),2) 优良率,
    -> round(100*(sum(case when score >= 90 then 1 else 0 end) / count(1)),2) 优秀率
    -> from sc
    -> left join course c on sc.cid = c.cid
    -> group by sc.cid,c.cname;
+----------+------------+-----------+-----------+-----------+-----------+-----------+-----------+-----------+
|  课程ID   | 课程name   | 最高分    |  最低分    | 平均分    | 及格率    |  中等率    |  优良率    | 优秀率    |
+----------+------------+-----------+-----------+-----------+-----------+-----------+-----------+-----------+
| 01       | 语文       |      80.0 |      31.0 |     64.50 |     66.67 |     50.00 |      0.00 |      0.00 |
| 02       | 数学       |      90.0 |      30.0 |     72.67 |     83.33 |     16.67 |     33.33 |     16.67 |
| 03       | 英语       |      99.0 |      20.0 |     68.50 |     66.67 |     33.33 |      0.00 |     33.33 |
+----------+------------+-----------+-----------+-----------+-----------+-----------+-----------+-----------+
3 rows in set (0.00 sec)

4.19、按各科成绩进行排序，并显示排名

select 
	s1.sid,
	s1.cid,
	s1.score,
	(select count(distinct score) from sc where cid = s1.cid and score >s1.score)+1 AS 排名 
from sc s1
order by s1.cid,4;

mysql> select 
    -> s1.sid,
    -> s1.cid,
    -> s1.score,
    -> (select count(distinct score) from sc where cid = s1.cid and score >s1.score)+1 AS 排名 
    -> from sc s1
    -> order by s1.cid,4;
+-----+-----+-------+--------+
| sid | cid | score | 排名   |
+-----+-----+-------+--------+
| 01  | 01  |  80.0 |      1 |
| 03  | 01  |  80.0 |      1 |
| 05  | 01  |  76.0 |      2 |
| 02  | 01  |  70.0 |      3 |
| 04  | 01  |  50.0 |      4 |
| 06  | 01  |  31.0 |      5 |
| 01  | 02  |  90.0 |      1 |
| 07  | 02  |  89.0 |      2 |
| 05  | 02  |  87.0 |      3 |
| 03  | 02  |  80.0 |      4 |
| 02  | 02  |  60.0 |      5 |
| 04  | 02  |  30.0 |      6 |
| 01  | 03  |  99.0 |      1 |
| 07  | 03  |  98.0 |      2 |
| 02  | 03  |  80.0 |      3 |
| 03  | 03  |  80.0 |      3 |
| 06  | 03  |  34.0 |      4 |
| 04  | 03  |  20.0 |      5 |
+-----+-----+-------+--------+
18 rows in set (0.00 sec)

select 
	stu.sname '学生姓名',
	c.cname '课程名',
	s1.score '分数',
	(select count(distinct score) from sc where cid = s1.cid and score >s1.score)+1 AS 排名 
from sc s1
join student stu on s1.sid = stu.sid
join course c on s1.cid = c.cid
order by s1.cid,4;

mysql> select 
    -> stu.sname '学生姓名',
    -> c.cname '课程名',
    -> s1.score '分数',
    -> (select count(distinct score) from sc where cid = s1.cid and score >s1.score)+1 AS 排名 
    -> from sc s1
    -> join student stu on s1.sid = stu.sid
    -> join course c on s1.cid = c.cid
    -> order by s1.cid,4;
+--------------+-----------+--------+--------+
| 学生姓名     | 课程名     | 分数   |  排名   |
+--------------+-----------+--------+--------+
| 赵雷         | 语文      |   80.0 |      1 |
| 孙风         | 语文      |   80.0 |      1 |
| 周梅         | 语文      |   76.0 |      2 |
| 钱电         | 语文      |   70.0 |      3 |
| 李云         | 语文      |   50.0 |      4 |
| 吴兰         | 语文      |   31.0 |      5 |
| 赵雷         | 数学      |   90.0 |      1 |
| 郑竹         | 数学      |   89.0 |      2 |
| 周梅         | 数学      |   87.0 |      3 |
| 孙风         | 数学      |   80.0 |      4 |
| 钱电         | 数学      |   60.0 |      5 |
| 李云         | 数学      |   30.0 |      6 |
| 赵雷         | 英语      |   99.0 |      1 |
| 郑竹         | 英语      |   98.0 |      2 |
| 孙风         | 英语      |   80.0 |      3 |
| 钱电         | 英语      |   80.0 |      3 |
| 吴兰         | 英语      |   34.0 |      4 |
| 李云         | 英语      |   20.0 |      5 |
+--------------+-----------+--------+--------+
18 rows in set (0.01 sec)

4.20、查询学生的总成绩并进行排名

select
	stu.sname '学生姓名',
	sum(score) '总成绩'
from sc
inner join student stu on sc.sid = stu.sid
group by sc.sid
order by 2 asc;

mysql> select
    -> stu.sname '学生姓名',
    -> sum(score) '总成绩'
    -> from sc
    -> inner join student stu on sc.sid = stu.sid
    -> group by sc.sid
    -> order by 2 asc;
+--------------+-----------+
| 学生姓名     | 总成绩    |
+--------------+-----------+
| 吴兰         |      65.0 |
| 李云         |     100.0 |
| 周梅         |     163.0 |
| 郑竹         |     187.0 |
| 钱电         |     210.0 |
| 孙风         |     240.0 |
| 赵雷         |     269.0 |
+--------------+-----------+
7 rows in set (0.00 sec)

select sid,sum(score) sscore from sc group by sid;

select
	stu.sname '学生姓名',
	t1.sscore '总成绩',
	(select count(distinct t2.sscore) 
	from (select sid,sum(score) sscore from sc group by sid) t2 
	where t2.sscore > t1.sscore
	)+1 '排名'
from student stu,(select sid,sum(score) sscore from sc group by sid) t1
where stu.sid = t1.sid
order by 3;

mysql> select
    -> stu.sname '学生姓名',
    -> t1.sscore '总成绩',
    -> (select count(distinct t2.sscore) 
    -> from (select sid,sum(score) sscore from sc group by sid) t2 
    -> where t2.sscore > t1.sscore
    -> )+1 '排名'
    -> from student stu,(select sid,sum(score) sscore from sc group by sid) t1
    -> where stu.sid = t1.sid
    -> order by 3;
+--------------+-----------+--------+
| 学生姓名     | 总成绩    | 排名   |
+--------------+-----------+--------+
| 赵雷         |     269.0 |      1 |
| 孙风         |     240.0 |      2 |
| 钱电         |     210.0 |      3 |
| 郑竹         |     187.0 |      4 |
| 周梅         |     163.0 |      5 |
| 李云         |     100.0 |      6 |
| 吴兰         |      65.0 |      7 |
+--------------+-----------+--------+
7 rows in set (0.00 sec)

4.21、查询不同老师所教不同课程平均分从高到低显示

select c.cid,c.cname,t.tname
from course c
join teacher t on c.tid = t.tid;

mysql> select c.cid,c.cname,t.tname
    -> from course c
    -> join teacher t on c.tid = t.tid;
+-----+--------+--------+
| cid | cname  | tname  |
+-----+--------+--------+
| 02  | 数学   | 张三   |
| 01  | 语文   | 李四   |
| 03  | 英语   | 王五   |
+-----+--------+--------+
3 rows in set (0.00 sec)

select cid,avg(score) from sc group by cid;

mysql> select cid,avg(score) from sc group by cid;
+-----+------------+
| cid | avg(score) |
+-----+------------+
| 01  |   64.50000 |
| 02  |   72.66667 |
| 03  |   68.50000 |
+-----+------------+
3 rows in set (0.00 sec)

select c.cid '课程ID',c.cname '课程名',t.tname '教师名',score.avgscore '课程均分'
from course c
join teacher t on c.tid = t.tid
join (select cid,avg(score) avgscore from sc group by cid) score on c.cid = score.cid
order by 4 desc;

mysql> select c.cid '课程ID',c.cname '课程名',t.tname '教师名',score.avgscore '课程均分'
    -> from course c
    -> join teacher t on c.tid = t.tid
    -> join (select cid,avg(score) avgscore from sc group by cid) score on c.cid = score.cid
    -> order by 4 desc;
+----------+-----------+-----------+--------------+
| 课程ID   | 课程名    | 教师名     | 课程均分     |
+----------+-----------+-----------+--------------+
| 02       | 数学      | 张三      |     72.66667 |
| 03       | 英语      | 王五      |     68.50000 |
| 01       | 语文      | 李四      |     64.50000 |
+----------+-----------+-----------+--------------+
3 rows in set (0.00 sec)

select c.cid '课程ID',c.cname '课程名',t.tname '教师名',round(avg(score),2) '课程均分'
from course c
left join teacher t on c.tid = t.tid
left join sc on c.cid = sc.cid
group by c.cid
order by 4 desc;

mysql> select c.cid '课程ID',c.cname '课程名',t.tname '教师名',round(avg(score),2) '课程均分'
    -> from course c
    -> left join teacher t on c.tid = t.tid
    -> left join sc on c.cid = sc.cid
    -> group by c.cid
    -> order by 4 desc;
+----------+-----------+-----------+--------------+
| 课程ID   | 课程名    | 教师名     | 课程均分     |
+----------+-----------+-----------+--------------+
| 02       | 数学      | 张三      |        72.67 |
| 03       | 英语      | 王五      |        68.50 |
| 01       | 语文      | 李四      |        64.50 |
+----------+-----------+-----------+--------------+
3 rows in set (0.00 sec)

4.22、查询所有课程的成绩第2名到第3名的学生信息及该课程成绩

SELECT a.* , (SELECT COUNT(DISTINCT score) FROM SC WHERE CID = a.CID AND score > a.score) + 1 AS 排名
FROM SC AS a HAVING 排名 BETWEEN 2 AND 3 ORDER BY a.CID,排名;

select
	s1.*,
	(select count(distinct score) from sc where cid = s1.cid and score > s1.score)+1 '排名'
from sc s1;

mysql> select
    -> s1.*,
    -> (select count(distinct score) from sc where cid = s1.cid and score > s1.score)+1 '排名'
    -> from sc s1;
+-----+-----+-------+--------+
| sid | cid | score | 排名   |
+-----+-----+-------+--------+
| 01  | 01  |  80.0 |      1 |
| 01  | 02  |  90.0 |      1 |
| 01  | 03  |  99.0 |      1 |
| 02  | 01  |  70.0 |      3 |
| 02  | 02  |  60.0 |      5 |
| 02  | 03  |  80.0 |      3 |
| 03  | 01  |  80.0 |      1 |
| 03  | 02  |  80.0 |      4 |
| 03  | 03  |  80.0 |      3 |
| 04  | 01  |  50.0 |      4 |
| 04  | 02  |  30.0 |      6 |
| 04  | 03  |  20.0 |      5 |
| 05  | 01  |  76.0 |      2 |
| 05  | 02  |  87.0 |      3 |
| 06  | 01  |  31.0 |      5 |
| 06  | 03  |  34.0 |      4 |
| 07  | 02  |  89.0 |      2 |
| 07  | 03  |  98.0 |      2 |
+-----+-----+-------+--------+
18 rows in set (0.00 sec)

select
	stu.sname '学生名字',
	c.cname '课程名',
	s1.score '分数',
	(select count(distinct score) from sc where cid = s1.cid and score > s1.score)+1 rank
from sc s1
inner join student stu on s1.sid = stu.sid
inner join course c on s1.cid = c.cid
having rank between 2 and 3
order by 4 desc;

mysql> select
    -> stu.sname '学生名字',
    -> c.cname '课程名',
    -> s1.score '分数',
    -> (select count(distinct score) from sc where cid = s1.cid and score > s1.score)+1 rank
    -> from sc s1
    -> inner join student stu on s1.sid = stu.sid
    -> inner join course c on s1.cid = c.cid
    -> having rank between 2 and 3
    -> order by 4 desc;
+--------------+-----------+--------+------+
| 学生名字     | 课程名     | 分数    | rank |
+--------------+-----------+--------+------+
| 钱电         | 英语      |   80.0 |    3 |
| 钱电         | 语文      |   70.0 |    3 |
| 孙风         | 英语      |   80.0 |    3 |
| 周梅         | 数学      |   87.0 |    3 |
| 郑竹         | 数学      |   89.0 |    2 |
| 周梅         | 语文      |   76.0 |    2 |
| 郑竹         | 英语      |   98.0 |    2 |
+--------------+-----------+--------+------+
7 rows in set (0.01 sec)

4.23、统计各科成绩各分数段人数：课程编号,课程名称,[100-85],[85-70],[70-60],[0-60]及所占百分比

select
	sc.cid '课程编号',
	sum(case when score >= 0 and score < 60 then 1 else 0 end) '0-60人数',
	round(100 * (sum(case when score >= 0 and score < 60 then 1 else 0 end) / count(*) ),2) '0-60占比'
from sc
group by sc.cid; -- 分课程统计

mysql> select
    -> sc.cid '课程编号',
    -> sum(case when score >= 0 and score < 60 then 1 else 0 end) '0-60人数',
    -> round(100 * (sum(case when score >= 0 and score < 60 then 1 else 0 end) / count(*) ),2) '0-60占比'
    -> from sc
    -> group by sc.cid; -- 分课程统计
+--------------+------------+------------+
| 课程编号     | 0-60人数   | 0-60占比   |
+--------------+------------+------------+
| 01           |          2 |      33.33 |
| 02           |          1 |      16.67 |
| 03           |          2 |      33.33 |
+--------------+------------+------------+
3 rows in set (0.00 sec)

select
	sc.cid '课程编号',
	c.cname '科目',
	count(*) '课程人数',
	sum(case when score >= 85 then 1 else 0 end) '85-100人数',
	round(100 * (sum(case when score > 85 then 1 else 0 end) / count(*) ),2) '85-100占比',
	sum(case when score < 85 and score >= 70 then 1 else 0 end) '70-85人数',
	round(100 * (sum(case when score < 85 and score >= 70 then 1 else 0 end) / count(*) ),2) '70-85占比',
	sum(case when score < 70 and score >= 60 then 1 else 0 end) '60-70人数',
	round(100 * (sum(case when score < 70 and score >= 60 then 1 else 0 end) / count(*) ),2) '60-70占比',
	sum(case when score >= 0 and score < 60 then 1 else 0 end) '0-60人数',
	round(100 * (sum(case when score >= 0 and score < 60 then 1 else 0 end) / count(*) ),2) '0-60占比'
from sc
left join course c on sc.cid = c.cid
group by sc.cid -- 分课程统计
order by sc.cid;

mysql> select
    -> sc.cid '课程编号',
    -> c.cname '科目',
    -> count(*) '课程人数',
    -> sum(case when score >= 85 then 1 else 0 end) '85-100人数',
    -> round(100 * (sum(case when score > 85 then 1 else 0 end) / count(*) ),2) '85-100占比',
    -> sum(case when score < 85 and score >= 70 then 1 else 0 end) '70-85人数',
    -> round(100 * (sum(case when score < 85 and score >= 70 then 1 else 0 end) / count(*) ),2) '70-85占比',
    -> sum(case when score < 70 and score >= 60 then 1 else 0 end) '60-70人数',
    -> round(100 * (sum(case when score < 70 and score >= 60 then 1 else 0 end) / count(*) ),2) '60-70占比',
    -> sum(case when score >= 0 and score < 60 then 1 else 0 end) '0-60人数',
    -> round(100 * (sum(case when score >= 0 and score < 60 then 1 else 0 end) / count(*) ),2) '0-60占比'
    -> from sc
    -> left join course c on sc.cid = c.cid
    -> group by sc.cid -- 分课程统计
    -> order by sc.cid;

+--------------+--------+--------------+--------------+--------------+-------------+-------------+-------------+-------------+------------+------------+
| 课程编号      | 科目   | 课程人数      | 85-100人数    | 85-100占比   | 70-85人数   | 70-85占比   | 60-70人数   | 60-70占比   | 0-60人数   | 0-60占比   |
+--------------+--------+--------------+--------------+--------------+-------------+-------------+-------------+-------------+------------+------------+
| 01           | 语文   |            6 |            0 |         0.00 |           4 |       66.67 |           0 |        0.00 |          2 |      33.33 |
| 02           | 数学   |            6 |            3 |        50.00 |           1 |       16.67 |           1 |       16.67 |          1 |      16.67 |
| 03           | 英语   |            6 |            2 |        33.33 |           2 |       33.33 |           0 |        0.00 |          2 |      33.33 |
+--------------+--------+--------------+--------------+--------------+-------------+-------------+-------------+-------------+------------+------------+
3 rows in set (0.01 sec)

4.24、查询学生平均成绩及其名次

select 
	sc.sid,
	avg(score) '平均成绩'
from sc
group by sc.sid

mysql> select 
    -> sc.sid,
    -> avg(score) '平均成绩'
    -> from sc
    -> group by sc.sid;
+-----+--------------+
| sid | 平均成绩     |
+-----+--------------+
| 01  |     89.66667 |
| 02  |     70.00000 |
| 03  |     80.00000 |
| 04  |     33.33333 |
| 05  |     81.50000 |
| 06  |     32.50000 |
| 07  |     93.50000 |
+-----+--------------+
7 rows in set (0.00 sec)

-- 查询平均成绩并去重
select distinct avg(score) avg_score from sc group by sid;

mysql> select distinct avg(score) avg_score from sc group by sid;
+-----------+
| avg_score |
+-----------+
|  89.66667 |
|  70.00000 |
|  80.00000 |
|  33.33333 |
|  81.50000 |
|  32.50000 |
|  93.50000 |
+-----------+
7 rows in set (0.00 sec)

select 
	stu.sid,
	t.avg_score '平均成绩',
	(select count(*) 
	from (select distinct avg(score) avg_score from sc group by sid) t2  -- 查询平均成绩并去重
	where t2.avg_score > t.avg_score
	)+1 '排名'
from student stu,(select avg(score) avg_score,sid from sc group by sid) t
where stu.sid = t.sid
group by stu.sid
order by 3;

mysql> select 
    -> stu.sid,
    -> t.avg_score '平均成绩',
    -> (select count(*) 
    -> from (select distinct avg(score) avg_score from sc group by sid) t2  -- 查询平均成绩并去重
    -> where t2.avg_score > t.avg_score
    -> )+1 '排名'
    -> from student stu,(select avg(score) avg_score,sid from sc group by sid) t
    -> where stu.sid = t.sid
    -> group by stu.sid
    -> order by 3;
+-----+--------------+--------+
| sid | 平均成绩     | 排名   |
+-----+--------------+--------+
| 07  |     93.50000 |      1 |
| 01  |     89.66667 |      2 |
| 05  |     81.50000 |      3 |
| 03  |     80.00000 |      4 |
| 02  |     70.00000 |      5 |
| 04  |     33.33333 |      6 |
| 06  |     32.50000 |      7 |
+-----+--------------+--------+
7 rows in set (0.00 sec)

select 
	stu.sid,
	t.avg_score '平均成绩',
	(select count(*) 
		from (select distinct avg(score) avg_score from sc group by sid) t2  -- 查询平均成绩并去重
		where t2.avg_score > t.avg_score
	)+1 '排名'
from student stu,(select avg(score) avg_score,sid from sc group by sid) t
where stu.sid = t.sid
group by stu.sid;

4.24.1、MySql中@符号的使用【经典】

@符号的作用：@后接变量名，用以定义一个变量，该变量的有效期为语句级，即再一次执行中始终有效

-- 由于通常情况下=被认为是等于比较运算符，因此赋值运算符一般使用:=
SELECT @lt:=1, @lt:=@lt+1, @lt:=@lt+1;

mysql> SELECT @lt:=1, @lt:=@lt+1, @lt:=@lt+1;
+--------+------------+------------+
| @lt:=1 | @lt:=@lt+1 | @lt:=@lt+1 |
+--------+------------+------------+
|      1 |          2 |          3 |
+--------+------------+------------+
1 row in set (0.00 sec)

select
	s.sid,  -- 学号
	@i:=@i+1 as '不保留空缺排名',   -- 直接i的自加
	@k:=(case when @avgscore=s.avg_score then @k else @i end) as '保留空缺排名',
	@avgscore:=avg_score as '平均分'  -- 表s中的值

from (select sid,round(avg(score), 2) as avg_score from sc
      group by sid
      order by 2 desc) s    -- 表s：平均成绩的排序和学号
      ,(select @avgscore:=0, @i:=0, @k:=0) b;   -- 表b：通过变量设置初始值

mysql> select
    -> s.sid,  -- 学号
    -> @i:=@i+1 as '不保留空缺排名',   -- 直接i的自加
    -> @k:=(case when @avgscore=s.avg_score then @k else @i end) as '保留空缺排名',
    -> @avgscore:=avg_score as '平均分'  -- 表s中的值
    -> 
    -> from (select sid,round(avg(score), 2) as avg_score from sc
    ->       group by sid
    ->       order by 2 desc) s    -- 表s：平均成绩的排序和学号
    ->       ,(select @avgscore:=0, @i:=0, @k:=0) b;   -- 表b：通过变量设置初始值
+-----+-----------------------+--------------------+-----------+
| sid | 不保留空缺排名          | 保留空缺排名        | 平均分    |
+-----+-----------------------+--------------------+-----------+
| 07  |                     1 |                  1 |     93.50 |
| 01  |                     2 |                  2 |     89.67 |
| 05  |                     3 |                  3 |     81.50 |
| 03  |                     4 |                  4 |     80.00 |
| 02  |                     5 |                  5 |     70.00 |
| 04  |                     6 |                  6 |     33.33 |
| 06  |                     7 |                  7 |     32.50 |
+-----+-----------------------+--------------------+-----------+
7 rows in set (0.00 sec)

4.25、查询各科成绩前三名的记录【经典】

先看
select *
from sc s1
inner join sc s2 on s1.cid = s2.cid and s1.score <= s2.score; -- 将分数表sc自连接，通过课程id对应起来，每个分数表s2的成绩最大

然后是

select s1.*,count(s2.score)
from sc s1
inner join sc s2 on s1.cid = s2.cid and s1.score <= s2.score
group by s1.sid,s1.cid,s1.score;

然后是

select s1.*,count(distinct s2.score)
from sc s1
inner join sc s2 on s1.cid = s2.cid and s1.score <= s2.score
group by s1.sid,s1.cid,s1.score;

最后是

select s1.*,count(distinct s2.score) rank
from sc s1
inner join sc s2 on s1.cid = s2.cid and s1.score <= s2.score
group by s1.sid,s1.cid,s1.score
having count(distinct s2.score) <=3
order by s1.cid,rank;

mysql> select s1.*,count(distinct s2.score) rank
    -> from sc s1
    -> inner join sc s2 on s1.cid = s2.cid and s1.score <= s2.score
    -> group by s1.sid,s1.cid,s1.score
    -> having count(distinct s2.score) <=3
    -> order by s1.cid,rank;
+-----+-----+-------+------+
| sid | cid | score | rank |
+-----+-----+-------+------+
| 03  | 01  |  80.0 |    1 |
| 01  | 01  |  80.0 |    1 |
| 05  | 01  |  76.0 |    2 |
| 02  | 01  |  70.0 |    3 |
| 01  | 02  |  90.0 |    1 |
| 07  | 02  |  89.0 |    2 |
| 05  | 02  |  87.0 |    3 |
| 01  | 03  |  99.0 |    1 |
| 07  | 03  |  98.0 |    2 |
| 02  | 03  |  80.0 |    3 |
| 03  | 03  |  80.0 |    3 |
+-----+-----+-------+------+
11 rows in set (0.00 sec)

4.26、查询每门课程被选修的学生数

select
	c.cid,
	c.cname,
	count(sc.cid)
from course c
inner join sc on c.cid = sc.cid
group by c.cid;

mysql> select
    -> c.cid,
    -> c.cname,
    -> count(sc.cid)
    -> from course c
    -> inner join sc on c.cid = sc.cid
    -> group by c.cid;
+-----+--------+---------------+
| cid | cname  | count(sc.cid) |
+-----+--------+---------------+
| 01  | 语文   |             6 |
| 02  | 数学   |             6 |
| 03  | 英语   |             6 |
+-----+--------+---------------+
3 rows in set (0.00 sec)

4.27、查询出只有两门课程的全部学生的学号和姓名

select
	stu.sid '学号',
	stu.sname '姓名'
from student stu
inner join sc on stu.sid = sc.sid
group by stu.sid
having count(sc.sid) = 2;

mysql> select
    -> stu.sid '学号',
    -> stu.sname '姓名'
    -> from student stu
    -> inner join sc on stu.sid = sc.sid
    -> group by stu.sid
    -> having count(sc.sid) = 2;
+--------+--------+
| 学号   | 姓名   |
+--------+--------+
| 05     | 周梅   |
| 06     | 吴兰   |
| 07     | 郑竹   |
+--------+--------+
3 rows in set (0.00 sec)

4.28、查询男生、女生人数

select
	ssex '性别',
	count(ssex) '人数'
from student
group by ssex;

mysql> select
    -> ssex '性别',
    -> count(ssex) '人数'
    -> from student
    -> group by ssex;
+--------+--------+
| 性别   | 人数   |
+--------+--------+
| 女     |      4 |
| 男     |      4 |
+--------+--------+
2 rows in set (0.00 sec)

select
	count(case when ssex = '男' then 1 else 0 end) '男',
	count(case when ssex = '女' then 1 else 0 end) '女'
from student;

mysql> select
    -> count(case when ssex = '男' then 1 else 0 end) '男',
    -> count(case when ssex = '女' then 1 else 0 end) '女'
    -> from student;
+-----+-----+
| 男  | 女  |
+-----+-----+
|   8 |   8 |
+-----+-----+
1 row in set (0.00 sec)

select count(ssex='女') '女',count(ssex='男') '男' from student;

mysql> select count(ssex='女') '女',count(ssex='男') '男' from student;
+-----+-----+
| 女  | 男  |
+-----+-----+
|   8 |   8 |
+-----+-----+
1 row in set (0.00 sec)

4.29、查询名字中含有"风"字的学生信息

select
	student.*
from student
where sname like '%风%';

mysql> select
    -> student.*
    -> from student
    -> where sname like '%风%';
+-----+--------+---------------------+------+
| sid | sname  | sage                | ssex |
+-----+--------+---------------------+------+
| 03  | 孙风   | 1990-05-20 00:00:00 | 男   |
+-----+--------+---------------------+------+
1 row in set (0.00 sec)

4.30、查询同名同性学生名单，并统计同名人数

INSERT INTO student VALUES('09' , '赵雷' , '2013-05-21' , '男'),('10' , '王菊' , '1314-12-25' , '女');

select
	s1.sname,
	s1.ssex,
	count(*)
from student s1
inner join student s2 on s1.sid <> s2.sid and s1.sname = s2.sname and s1.ssex = s2.ssex
group by s1.sname,s1.ssex;

mysql> INSERT INTO student VALUES('09' , '赵雷' , '2013-05-21' , '男'),('10' , '王菊' , '1314-12-25' , '女');
Query OK, 2 rows affected (0.01 sec)
Records: 2  Duplicates: 0  Warnings: 0

mysql> select
    -> s1.sname,
    -> s1.ssex,
    -> count(*)
    -> from student s1
    -> inner join student s2 on s1.sid <> s2.sid and s1.sname = s2.sname and s1.ssex = s2.ssex
    -> group by s1.sname,s1.ssex;
+--------+------+----------+
| sname  | ssex | count(*) |
+--------+------+----------+
| 王菊   | 女   |        2 |
| 赵雷   | 男   |        2 |
+--------+------+----------+
2 rows in set (0.00 sec)

mysql> delete from student where sid in ('09','10');
Query OK, 2 rows affected (0.00 sec)

delete from student where sid in ('09','10');

4.31、查询1990年出生的学生名单

select
	student.*
from student
where year(sage) = 1990;

mysql> select
    -> student.*
    -> from student
    -> where year(sage) = 1990;
+-----+--------+---------------------+------+
| sid | sname  | sage                | ssex |
+-----+--------+---------------------+------+
| 01  | 赵雷   | 1990-01-01 00:00:00 | 男   |
| 02  | 钱电   | 1990-12-21 00:00:00 | 男   |
| 03  | 孙风   | 1990-05-20 00:00:00 | 男   |
| 04  | 李云   | 1990-08-06 00:00:00 | 男   |
| 08  | 王菊   | 1990-01-20 00:00:00 | 女   |
+-----+--------+---------------------+------+
5 rows in set (0.00 sec)

select
	student.*
from student
where sage like '1990%';

mysql> select
    -> student.*
    -> from student
    -> where sage like '1990%';
+-----+--------+---------------------+------+
| sid | sname  | sage                | ssex |
+-----+--------+---------------------+------+
| 01  | 赵雷   | 1990-01-01 00:00:00 | 男   |
| 02  | 钱电   | 1990-12-21 00:00:00 | 男   |
| 03  | 孙风   | 1990-05-20 00:00:00 | 男   |
| 04  | 李云   | 1990-08-06 00:00:00 | 男   |
| 08  | 王菊   | 1990-01-20 00:00:00 | 女   |
+-----+--------+---------------------+------+
5 rows in set, 1 warning (0.00 sec)

4.32、查询每门课程的平均成绩，结果按平均成绩降序排列，平均成绩相同时，按课程编号升序排列

select
	sc.cid '课程编号',
	avg(score) '平均成绩'
from sc
group by cid
order by 2 desc,1 asc;

mysql> select
    -> sc.cid '课程编号',
    -> avg(score) '平均成绩'
    -> from sc
    -> group by cid
    -> order by 2 desc,1 asc;
+--------------+--------------+
| 课程编号     | 平均成绩     |
+--------------+--------------+
| 02           |     72.66667 |
| 03           |     68.50000 |
| 01           |     64.50000 |
+--------------+--------------+
3 rows in set (0.00 sec)

4.33、查询平均成绩大于等于85的所有学生的学号、姓名和平均成绩

select
	stu.sid '学号',
	stu.sname '姓名',
	avg(score) '平均成绩'
from sc
join student stu on sc.sid = stu.sid
group by sc.sid
having avg(score) >= 85;

mysql> select
    -> stu.sid '学号',
    -> stu.sname '姓名',
    -> avg(score) '平均成绩'
    -> from sc
    -> join student stu on sc.sid = stu.sid
    -> group by sc.sid
    -> having avg(score) >= 85;
+--------+--------+--------------+
| 学号   | 姓名   | 平均成绩     |
+--------+--------+--------------+
| 01     | 赵雷   |     89.66667 |
| 07     | 郑竹   |     93.50000 |
+--------+--------+--------------+
2 rows in set (0.00 sec)

4.34、查询课程名称为"数学"，且分数低于60的学生姓名和分数

select
	stu.sname '学生姓名',
	sc.score '分数'
from sc
inner join student stu on sc.sid = stu.sid
inner join course c on sc.cid = c.cid and c.cname = '数学'
where sc.score < 60;

mysql> select
    -> stu.sname '学生姓名',
    -> sc.score '分数'
    -> from sc
    -> inner join student stu on sc.sid = stu.sid
    -> inner join course c on sc.cid = c.cid and c.cname = '数学'
    -> where sc.score < 60;
+--------------+--------+
| 学生姓名     | 分数   |
+--------------+--------+
| 李云         |   30.0 |
+--------------+--------+
1 row in set (0.00 sec)

4.35、查询所有学生的课程及分数情况；

select
	stu.sid,
	stu.sname,
	sum(case when sc.cid = '01' then sc.score else 0 end) '语文',
	sum(case when sc.cid = '02' then sc.score else 0 end) '数学',
	sum(case when sc.cid = '03' then sc.score else 0 end) '英语',
	sum(sc.score)
from student stu
left join sc on stu.sid = sc.sid
group by stu.sid,stu.sname;

mysql> select
    -> stu.sid,
    -> stu.sname,
    -> sum(case when sc.cid = '01' then sc.score else 0 end) '语文',
    -> sum(case when sc.cid = '02' then sc.score else 0 end) '数学',
    -> sum(case when sc.cid = '03' then sc.score else 0 end) '英语',
    -> sum(sc.score)
    -> from student stu
    -> left join sc on stu.sid = sc.sid
    -> group by stu.sid,stu.sname;
+-----+--------+--------+--------+--------+---------------+
| sid | sname  | 语文   | 数学   | 英语   | sum(sc.score) |
+-----+--------+--------+--------+--------+---------------+
| 01  | 赵雷   |   80.0 |   90.0 |   99.0 |         269.0 |
| 02  | 钱电   |   70.0 |   60.0 |   80.0 |         210.0 |
| 03  | 孙风   |   80.0 |   80.0 |   80.0 |         240.0 |
| 04  | 李云   |   50.0 |   30.0 |   20.0 |         100.0 |
| 05  | 周梅   |   76.0 |   87.0 |    0.0 |         163.0 |
| 06  | 吴兰   |   31.0 |    0.0 |   34.0 |          65.0 |
| 07  | 郑竹   |    0.0 |   89.0 |   98.0 |         187.0 |
| 08  | 王菊   |    0.0 |    0.0 |    0.0 |          NULL |
+-----+--------+--------+--------+--------+---------------+
8 rows in set (0.00 sec)

select
	stu.sid,
	stu.sname,
	sum(case c.cname when '语文' then sc.score else 0 end) '语文',
	sum(case c.cname when '数学' then sc.score else 0 end) '数学',
	sum(case c.cname when '英语' then sc.score else 0 end) '英语',
	sum(sc.score)
from student stu
left join sc on stu.sid = sc.sid
left join course c on sc.cid = c.cid
group by stu.sid;

mysql> select
    -> stu.sid,
    -> stu.sname,
    -> sum(case c.cname when '语文' then sc.score else 0 end) '语文',
    -> sum(case c.cname when '数学' then sc.score else 0 end) '数学',
    -> sum(case c.cname when '英语' then sc.score else 0 end) '英语',
    -> sum(sc.score)
    -> from student stu
    -> left join sc on stu.sid = sc.sid
    -> left join course c on sc.cid = c.cid
    -> group by stu.sid;
+-----+--------+--------+--------+--------+---------------+
| sid | sname  | 语文   | 数学   | 英语   | sum(sc.score) |
+-----+--------+--------+--------+--------+---------------+
| 01  | 赵雷   |   80.0 |   90.0 |   99.0 |         269.0 |
| 02  | 钱电   |   70.0 |   60.0 |   80.0 |         210.0 |
| 03  | 孙风   |   80.0 |   80.0 |   80.0 |         240.0 |
| 04  | 李云   |   50.0 |   30.0 |   20.0 |         100.0 |
| 05  | 周梅   |   76.0 |   87.0 |    0.0 |         163.0 |
| 06  | 吴兰   |   31.0 |    0.0 |   34.0 |          65.0 |
| 07  | 郑竹   |    0.0 |   89.0 |   98.0 |         187.0 |
| 08  | 王菊   |    0.0 |    0.0 |    0.0 |          NULL |
+-----+--------+--------+--------+--------+---------------+
8 rows in set (0.00 sec)

4.36、查询任何一门课程成绩在70分以上的姓名、课程名称和分数

select
	stu.sname,
	c.cname,
	sc.score
from sc
join student stu on sc.sid = stu.sid
join course c on sc.cid = c.cid
having score > 70;

mysql> select
    -> stu.sname,
    -> c.cname,
    -> sc.score
    -> from sc
    -> join student stu on sc.sid = stu.sid
    -> join course c on sc.cid = c.cid
    -> having score > 70;
+--------+--------+-------+
| sname  | cname  | score |
+--------+--------+-------+
| 赵雷   | 语文   |  80.0 |
| 赵雷   | 数学   |  90.0 |
| 赵雷   | 英语   |  99.0 |
| 钱电   | 英语   |  80.0 |
| 孙风   | 语文   |  80.0 |
| 孙风   | 数学   |  80.0 |
| 孙风   | 英语   |  80.0 |
| 周梅   | 语文   |  76.0 |
| 周梅   | 数学   |  87.0 |
| 郑竹   | 数学   |  89.0 |
| 郑竹   | 英语   |  98.0 |
+--------+--------+-------+
11 rows in set (0.00 sec)

4.37、查询不及格的课程

select
	sc.sid,
	c.cname,
	sc.score
from sc
join course c on sc.cid = c.cid 
where sc.score < 60;

mysql> select
    -> sc.sid,
    -> c.cname,
    -> sc.score
    -> from sc
    -> join course c on sc.cid = c.cid 
    -> where sc.score < 60;
+-----+--------+-------+
| sid | cname  | score |
+-----+--------+-------+
| 04  | 语文   |  50.0 |
| 04  | 数学   |  30.0 |
| 04  | 英语   |  20.0 |
| 06  | 语文   |  31.0 |
| 06  | 英语   |  34.0 |
+-----+--------+-------+
5 rows in set (0.00 sec)

4.38、查询课程编号为01且课程成绩在80分及以上的学生的学号和姓名

select
	stu.sid,
	stu.sname,
	sc.score
from sc
join student stu on sc.sid = stu.sid
where sc.score >= 80 and sc.cid = '01';

mysql> select
    -> stu.sid,
    -> stu.sname,
    -> sc.score
    -> from sc
    -> join student stu on sc.sid = stu.sid
    -> where sc.score >= 80 and sc.cid = '01';
+-----+--------+-------+
| sid | sname  | score |
+-----+--------+-------+
| 01  | 赵雷   |  80.0 |
| 03  | 孙风   |  80.0 |
+-----+--------+-------+
2 rows in set (0.00 sec)

4.39、求每门课程的学生人数

select
	c.cname '课程名',
	count(*) '学生人数'
from course c
join sc on c.cid = sc.cid
group by c.cid;

mysql> select
    -> c.cname '课程名',
    -> count(*) '学生人数'
    -> from course c
    -> join sc on c.cid = sc.cid
    -> group by c.cid;
+-----------+--------------+
| 课程名    | 学生人数     |
+-----------+--------------+
| 语文      |            6 |
| 数学      |            6 |
| 英语      |            6 |
+-----------+--------------+
3 rows in set (0.01 sec)

4.40、查询选修"张三"老师所授课程的学生中，成绩最高的学生信息及其成绩

select
	stu.sid,
	stu.sname,
	sc.score,
	c.cname,
	t.tname
from sc
left join course c on c.cid = sc.cid
left join teacher t on c.tid = t.tid
left join student stu on sc.sid = stu.sid
where t.tname = '张三';

mysql> select
    -> stu.sid,
    -> stu.sname,
    -> sc.score,
    -> c.cname,
    -> t.tname
    -> from sc
    -> left join course c on c.cid = sc.cid
    -> left join teacher t on c.tid = t.tid
    -> left join student stu on sc.sid = stu.sid
    -> where t.tname = '张三';
+------+--------+-------+--------+--------+
| sid  | sname  | score | cname  | tname  |
+------+--------+-------+--------+--------+
| 01   | 赵雷   |  90.0 | 数学   | 张三   |
| 02   | 钱电   |  60.0 | 数学   | 张三   |
| 03   | 孙风   |  80.0 | 数学   | 张三   |
| 04   | 李云   |  30.0 | 数学   | 张三   |
| 05   | 周梅   |  87.0 | 数学   | 张三   |
| 07   | 郑竹   |  89.0 | 数学   | 张三   |
+------+--------+-------+--------+--------+
6 rows in set (0.00 sec)

select
	stu.sid,
	stu.sname,
	max(sc.score),
	c.cname,
	t.tname
from sc
left join course c on c.cid = sc.cid
left join teacher t on c.tid = t.tid
left join student stu on sc.sid = stu.sid
where t.tname = '张三';

mysql> select
    -> stu.sid,
    -> stu.sname,
    -> max(sc.score),
    -> c.cname,
    -> t.tname
    -> from sc
    -> left join course c on c.cid = sc.cid
    -> left join teacher t on c.tid = t.tid
    -> left join student stu on sc.sid = stu.sid
    -> where t.tname = '张三';
+------+--------+---------------+--------+--------+
| sid  | sname  | max(sc.score) | cname  | tname  |
+------+--------+---------------+--------+--------+
| 01   | 赵雷   |          90.0 | 数学   | 张三   |
+------+--------+---------------+--------+--------+
1 row in set (0.00 sec)

4.41、查询不同课程成绩相同的学生的学生编号、课程编号、学生成绩

select
	distinct s1.sid,
	s1.cid,
	s1.score
from sc s1
join sc s2 on s1.score = s2.score and s1.cid != s2.cid and s1.sid != s2.sid;

mysql> select
    -> distinct s1.sid,
    -> s1.cid,
    -> s1.score
    -> from sc s1
    -> join sc s2 on s1.score = s2.score and s1.cid != s2.cid and s1.sid != s2.sid;
+-----+-----+-------+
| sid | cid | score |
+-----+-----+-------+
| 02  | 03  |  80.0 |
| 03  | 02  |  80.0 |
| 03  | 03  |  80.0 |
| 01  | 01  |  80.0 |
| 03  | 01  |  80.0 |
+-----+-----+-------+
5 rows in set (0.00 sec)

-- 查询成绩相同的分数
select score from sc group by score having count(1)>1;

select
	sc.sid,
	sc.cid,
	sc.score
from sc
where sc.score in (select score from sc group by score having count(1)>1);

mysql> select
    -> sc.sid,
    -> sc.cid,
    -> sc.score
    -> from sc
    -> where sc.score in (select score from sc group by score having count(1)>1);
+-----+-----+-------+
| sid | cid | score |
+-----+-----+-------+
| 01  | 01  |  80.0 |
| 02  | 03  |  80.0 |
| 03  | 01  |  80.0 |
| 03  | 02  |  80.0 |
| 03  | 03  |  80.0 |
+-----+-----+-------+
5 rows in set (0.00 sec)

4.42、查询每门功课成绩最好的前两名【经典】

select
	s1.*,
	count(distinct(s2.score)) rank
from sc s1
join sc s2 on s1.cid = s2.cid and s1.score <= s2.score
group by s1.sid,s1.cid,s1.score
having rank <=2
order by s1.cid,rank;

mysql> select
    -> s1.*,
    -> count(distinct(s2.score)) rank
    -> from sc s1
    -> join sc s2 on s1.cid = s2.cid and s1.score <= s2.score
    -> group by s1.sid,s1.cid,s1.score
    -> having rank <=2
    -> order by s1.cid,rank;
+-----+-----+-------+------+
| sid | cid | score | rank |
+-----+-----+-------+------+
| 01  | 01  |  80.0 |    1 |
| 03  | 01  |  80.0 |    1 |
| 05  | 01  |  76.0 |    2 |
| 01  | 02  |  90.0 |    1 |
| 07  | 02  |  89.0 |    2 |
| 01  | 03  |  99.0 |    1 |
| 07  | 03  |  98.0 |    2 |
+-----+-----+-------+------+
7 rows in set (0.00 sec)

select
	stu.sname,
	c.cname,
	t.tname,
	s1.score,
	count(distinct(s2.score)) rank
from sc s1
join sc s2 on s1.cid = s2.cid and s1.score <= s2.score
join student stu on s1.sid = stu.sid
join course c on s1.cid = c.cid
join teacher t on c.tid = t.tid
group by s1.sid,s1.cid,s1.score
having rank <=2
order by s1.cid,rank;

mysql> select
    -> stu.sname,
    -> c.cname,
    -> t.tname,
    -> s1.score,
    -> count(distinct(s2.score)) rank
    -> from sc s1
    -> join sc s2 on s1.cid = s2.cid and s1.score <= s2.score
    -> join student stu on s1.sid = stu.sid
    -> join course c on s1.cid = c.cid
    -> join teacher t on c.tid = t.tid
    -> group by s1.sid,s1.cid,s1.score
    -> having rank <=2
    -> order by s1.cid,rank;
+--------+--------+--------+-------+------+
| sname  | cname  | tname  | score | rank |
+--------+--------+--------+-------+------+
| 孙风   | 语文   | 李四   |  80.0 |    1 |
| 赵雷   | 语文   | 李四   |  80.0 |    1 |
| 周梅   | 语文   | 李四   |  76.0 |    2 |
| 赵雷   | 数学   | 张三   |  90.0 |    1 |
| 郑竹   | 数学   | 张三   |  89.0 |    2 |
| 赵雷   | 英语   | 王五   |  99.0 |    1 |
| 郑竹   | 英语   | 王五   |  98.0 |    2 |
+--------+--------+--------+-------+------+
7 rows in set (0.00 sec)

select
	s1.*
from sc s1
where (select count(1)   -- count(1)类似count(*):统计表s2中分数大的数量
       from sc s2
       where s2.cid=s1.cid   -- 课程相同
       and s2.score >= s1.score) <= 2   -- 前2名
order by s1.cid;

此方法有缺陷，重复分数未统计

mysql> select
    -> s1.*
    -> from sc s1
    -> where (select count(1)   -- count(1)类似count(*):统计表s2中分数大的数量
    ->        from sc s2
    ->        where s2.cid=s1.cid   -- 课程相同
    ->        and s2.score >= s1.score) <= 2   -- 前2名
    -> order by s1.cid;
+-----+-----+-------+
| sid | cid | score |
+-----+-----+-------+
| 01  | 01  |  80.0 |
| 03  | 01  |  80.0 |
| 01  | 02  |  90.0 |
| 07  | 02  |  89.0 |
| 01  | 03  |  99.0 |
| 07  | 03  |  98.0 |
+-----+-----+-------+
6 rows in set (0.00 sec)

select
	s1.*
from sc s1
where (select count(distinct(s2.score))   -- 根据分数去重并统计表s2中分数大的数量
       from sc s2
       where s2.cid=s1.cid   -- 课程相同
       and s2.score >= s1.score) <= 2   -- 前2名
order by s1.cid;

mysql> select
    -> s1.*
    -> from sc s1
    -> where (select count(distinct(s2.score))   -- 根据分数去重并统计表s2中分数大的数量
    ->        from sc s2
    ->        where s2.cid=s1.cid   -- 课程相同
    ->        and s2.score >= s1.score) <= 2   -- 前2名
    -> order by s1.cid;
+-----+-----+-------+
| sid | cid | score |
+-----+-----+-------+
| 01  | 01  |  80.0 |
| 03  | 01  |  80.0 |
| 05  | 01  |  76.0 |
| 01  | 02  |  90.0 |
| 07  | 02  |  89.0 |
| 01  | 03  |  99.0 |
| 07  | 03  |  98.0 |
+-----+-----+-------+
7 rows in set (0.00 sec)

4.43、统计每门课程的学生选修人数（超过5人的课程才统计）；要求输出课程号和选修人数，查询结果按人数降序排列。

select
	sc.cid '课程号',
	c.cname '课程名',
	count(*) '选修人数'
from sc
join course c on sc.cid = c.cid
group by sc.cid
having(count(*)) > 5;

mysql> select
    -> sc.cid '课程号',
    -> c.cname '课程名',
    -> count(*) '选修人数'
    -> from sc
    -> join course c on sc.cid = c.cid
    -> group by sc.cid
    -> having(count(*)) > 5;
+-----------+-----------+--------------+
| 课程号    | 课程名    | 选修人数     |
+-----------+-----------+--------------+
| 01        | 语文      |            6 |
| 02        | 数学      |            6 |
| 03        | 英语      |            6 |
+-----------+-----------+--------------+
3 rows in set (0.00 sec)

4.44、检索至少选修两门课程的学生学号

select
	sc.sid,
	stu.sname,
	count(*)
from sc
join student stu on sc.sid = stu.sid
group by sc.sid
having count(*) >= 2;

mysql> select
    -> sc.sid,
    -> stu.sname,
    -> count(*)
    -> from sc
    -> join student stu on sc.sid = stu.sid
    -> group by sc.sid
    -> having count(*) >= 2;
+-----+--------+----------+
| sid | sname  | count(*) |
+-----+--------+----------+
| 01  | 赵雷   |        3 |
| 02  | 钱电   |        3 |
| 03  | 孙风   |        3 |
| 04  | 李云   |        3 |
| 05  | 周梅   |        2 |
| 06  | 吴兰   |        2 |
| 07  | 郑竹   |        2 |
+-----+--------+----------+
7 rows in set (0.00 sec)

4.45、查询选修了全部课程的学生信息

select
	sc.sid,
	stu.sname,
	count(*)
from sc
join student stu on sc.sid = stu.sid
group by sc.sid
having count(*) = (select count(*) from course);

mysql> select
    -> sc.sid,
    -> stu.sname,
    -> count(*)
    -> from sc
    -> join student stu on sc.sid = stu.sid
    -> group by sc.sid
    -> having count(*) = (select count(*) from course);
+-----+--------+----------+
| sid | sname  | count(*) |
+-----+--------+----------+
| 01  | 赵雷   |        3 |
| 02  | 钱电   |        3 |
| 03  | 孙风   |        3 |
| 04  | 李云   |        3 |
+-----+--------+----------+
4 rows in set (0.00 sec)

4.46、查询各学生的年龄

-- 通过date_format函数能够指定返回的数据
select year(now());
select date_format(now(), '%Y');
select date_format(now(), '%Y-%m-%d');

mysql> select year(now());
+-------------+
| year(now()) |
+-------------+
|        2024 |
+-------------+
1 row in set (0.00 sec)

mysql> select date_format(now(), '%Y');
+--------------------------+
| date_format(now(), '%Y') |
+--------------------------+
| 2024                     |
+--------------------------+
1 row in set (0.00 sec)

mysql> select date_format(now(), '%Y-%m-%d');
+--------------------------------+
| date_format(now(), '%Y-%m-%d') |
+--------------------------------+
| 2024-02-08                     |
+--------------------------------+
1 row in set (0.00 sec)

select
	stu.sname '姓名',
	TIMESTAMPDIFF(year,stu.sage,now()) '年龄'  -- TIMESTAMPDIFF函数返回年差
from student stu;

mysql> select
    -> stu.sname '姓名',
    -> TIMESTAMPDIFF(year,stu.sage,now()) '年龄'
    -> from student stu;
+--------+--------+
| 姓名   | 年龄   |
+--------+--------+
| 赵雷   |     34 |
| 钱电   |     33 |
| 孙风   |     33 |
| 李云   |     33 |
| 周梅   |     32 |
| 吴兰   |     31 |
| 郑竹   |     34 |
| 王菊   |     34 |
+--------+--------+
8 rows in set (0.00 sec)

select
	sname
	,sage
	,date_format(now(), '%Y') - date_format(sage, '%Y') - (case when date_format(now(), '%m%d') > date_format(sage, '%m%d') then 0 else 1 end) as age  -- 当前日期大，说明已经过生了，年龄正常；反之说明今年还没有到年龄-1
from student;

mysql> select
    -> sname
    -> ,sage
    -> ,date_format(now(), '%Y') - date_format(sage, '%Y') - (case when date_format(now(), '%m%d') > date_format(sage, '%m%d') then 0 else 1 end) as age  -- 当前日 期大，说明已经过生了，年龄正常；反之说明今年还没有到年龄-1
    -> from student;
+--------+---------------------+------+
| sname  | sage                | age  |
+--------+---------------------+------+
| 赵雷   | 1990-01-01 00:00:00 |   34 |
| 钱电   | 1990-12-21 00:00:00 |   33 |
| 孙风   | 1990-05-20 00:00:00 |   33 |
| 李云   | 1990-08-06 00:00:00 |   33 |
| 周梅   | 1991-12-01 00:00:00 |   32 |
| 吴兰   | 1992-03-01 00:00:00 |   31 |
| 郑竹   | 1989-07-01 00:00:00 |   34 |
| 王菊   | 1990-01-20 00:00:00 |   34 |
+--------+---------------------+------+
8 rows in set (0.00 sec)

4.47、查询本周过生日的学生

INSERT INTO student VALUES('09' , '李白' , date_format(now(),'%Y-%m-%d'), '男'),
('10' , '玛利亚' , date_format(now(),'%Y-%m-%d'), '女');

select * from student;

mysql> INSERT INTO student VALUES('09' , '李白' , date_format(now(),'%Y-%m-%d'), '男'),
    -> ('10' , '玛利亚' , date_format(now(),'%Y-%m-%d'), '女');
Query OK, 2 rows affected (0.00 sec)
Records: 2  Duplicates: 0  Warnings: 0

mysql> 
mysql> select * from student;
+-----+-----------+---------------------+------+
| sid | sname     | sage                | ssex |
+-----+-----------+---------------------+------+
| 01  | 赵雷      | 1990-01-01 00:00:00 | 男   |
| 02  | 钱电      | 1990-12-21 00:00:00 | 男   |
| 03  | 孙风      | 1990-05-20 00:00:00 | 男   |
| 04  | 李云      | 1990-08-06 00:00:00 | 男   |
| 05  | 周梅      | 1991-12-01 00:00:00 | 女   |
| 06  | 吴兰      | 1992-03-01 00:00:00 | 女   |
| 07  | 郑竹      | 1989-07-01 00:00:00 | 女   |
| 08  | 王菊      | 1990-01-20 00:00:00 | 女   |
| 09  | 李白      | 2024-02-08 00:00:00 | 男   |
| 10  | 玛利亚    | 2024-02-08 00:00:00 | 女   |
+-----+-----------+---------------------+------+
10 rows in set (0.01 sec)

select
	stu.*
from student stu
where week(date_format(now(),'%Y%m%d')) = week(sage);

mysql> select
    -> stu.*
    -> from student stu
    -> where week(date_format(now(),'%Y%m%d')) = week(sage);
+-----+-----------+---------------------+------+
| sid | sname     | sage                | ssex |
+-----+-----------+---------------------+------+
| 09  | 李白       | 2024-02-08 00:00:00 | 男   |
| 10  | 玛利亚     | 2024-02-08 00:00:00 | 女   |
+-----+-----------+---------------------+------+
2 rows in set (0.00 sec)

select * from student where WEEKOFYEAR(NOW())-WEEKOFYEAR(sage)=0;

mysql> select * from student where WEEKOFYEAR(NOW())-WEEKOFYEAR(sage)=0;
+-----+-----------+---------------------+------+
| sid | sname     | sage                | ssex |
+-----+-----------+---------------------+------+
| 09  | 李白      | 2024-02-08 00:00:00 | 男   |
| 10  | 玛利亚    | 2024-02-08 00:00:00 | 女   |
+-----+-----------+---------------------+------+
2 rows in set (0.00 sec)

4.48、查询下周过生日的学生

select
	stu.*
from student stu
where week(date_format(now(),'%Y%m%d'))+1 = week(sage);

select * from student where WEEKOFYEAR(NOW())-WEEKOFYEAR(sage)=-1;

4.49、查询本月过生日的学生

select
	stu.*
from student stu
where month(date_format(now(),'%Y%m%d')) = month(sage);

mysql> select
    -> stu.*
    -> from student stu
    -> where month(date_format(now(),'%Y%m%d')) = month(sage);
+-----+-----------+---------------------+------+
| sid | sname     | sage                | ssex |
+-----+-----------+---------------------+------+
| 09  | 李白       | 2024-02-08 00:00:00 | 男   |
| 10  | 玛利亚     | 2024-02-08 00:00:00 | 女   |
+-----+-----------+---------------------+------+
2 rows in set (0.00 sec)

4.50、查询下月过生日的学生

select
	stu.*
from student stu
where month(date_format(now(),'%Y%m%d'))+1 = month(sage);

mysql> select
    -> stu.*
    -> from student stu
    -> where month(date_format(now(),'%Y%m%d'))+1 = month(sage);
+-----+--------+---------------------+------+
| sid | sname  | sage                | ssex |
+-----+--------+---------------------+------+
| 06  | 吴兰   | 1992-03-01 00:00:00 | 女   |
+-----+--------+---------------------+------+
1 row in set (0.00 sec)

-- 假设现在是12月份，那么下个月就是明年的1月份,解决：当前月份与12的余数
select
	stu.*
from student stu
where mod(month(now()),12)+1 = month(sage);

mysql> select
    -> stu.*
    -> from student stu
    -> where mod(month(now()),12)+1 = month(sage);
+-----+--------+---------------------+------+
| sid | sname  | sage                | ssex |
+-----+--------+---------------------+------+
| 06  | 吴兰    | 1992-03-01 00:00:00 | 女   |
+-----+--------+---------------------+------+
1 row in set (0.00 sec)

end