半天学会MySQL
MySQL的基础知识
数据库基础知识
1.为什么要使用数据库
-
~ 解决数据持久化问题
- 高效的管理数据（增删改查） ### 2.数据库的分类：
- 关系型数据库
- 理论基础：关系代数
- 具体表象：二维表
~ 行：记录
~ 列：字段
- 编程语言：SQL（结构化查询语言）
~ DDL - 数据定义语言 —> create / drop / alter
~ DML - 数据操作语言 —> insert / delete / update
~ DQL - 数据查询语言 —> select
-
~ DCL - 数据控制语言 —> grant / revoke
- 非关系型数据库（NoSQL / NewSQL）
- MongoDB —> 文档数据库
- Redis —> KV数据库
3.相关术语
-
~ DB - database
-
DBA - database administrator
-
DBMS - database management system
-
DBS - database system
### 4.使用MySQL
-
查看所有数据库：
show databases; -
切换数据库：
use school; -
查看数据库下所有的表：
show tables; -
查看所有的字符集（编码）：
show charset; -
查看所有的校对（排序）规则：
show collation; -
查看所有的引擎：
show engines; -
创建/删除数据库：
create schema school default charset utf8mb4;
create database school default charset utf8mb4 collate utf8mb4_bin;
create database school default character set utf8mb4;
drop database if exists school;
~ 创建/删除/修改二维表：
create table tb_student (列名 数据类型 约束条件);
drop table tb_student;
alter table tb_student add column ...;
alter table tb_student drop column ...;
alter table tb_student modify column ...;
alter table tb_student change column ...;
alter table tb_student add constraint ...;
alter table tb_student drop constraint ...;
- 主键（primary key）：能够唯一确定一条记录的字段。
- 外键（foreign key）：外来的主键（其他表的主键）。
~ 插入/删除/更新数据：
insert into tb_student (...) values (...);
insert into tb_student (...) values (...), (...), (...);
delete from tb_student where stu_id=...;
delete from tb_student where stu_id in (...);
update tb_student set ..., ... where stu_id=...;
5.MySQL的数据类型 —> ? data types;
~ 整数：
- int / integer —> -2^31 ~ 2^31 - 1
- int unsigned —> 0 ~ 2^32 - 1
- bigint —> -2^63 ~ 2^63 - 1
- bigint unsigned —> 0 ~ 2^64 - 1
- smallint —> -32768 ~ 32767
- smallint unsigned —> 0 ~ 65535
- tinyint —> -128 ~ 127
-
- tinyint unsigned —> 0 ~ 255
- 小数：
- float / double
- decimal —> decimal(10,2)
~ 字符串：
- char(N) / varchar(N)
-
- longtext —> longblob（二进制大对象）—> 4G —> 放资源路径（URL）
- 时间日期：
- date / time / datetime(6)
-
- timestamp —> 1970-1-1 00:00:00
- 布尔型：
- boolean —> tinyint(1) —> 0 / 1
MySQL常用命令
在powershell窗口中输入 mysql -u root -p 然后输入下列命令，前提得保证MySQL服务器是打开的，没打开请手动打开。
-
查看所有数据库
show databases; -
查看所有字符集（编码方式）
show character set; -
查看所有的校对规则（排序）
show collation; -
查看所有的引擎
show engines; -
查看所有的日志文件
show binary logs; -
查看数据库下所有的表
show tables; -
获取帮助
1.查看show命令的帮助。
？show2.查看有哪些帮助内容
？contents3.获取函数的帮助
？functions4.获取数据类型的帮助
？data types
SQL 的四种语言
DDL（数据定义语言）
-- 如果存在名为school的数据库就删除它
drop database if exists `school`;
-- 创建名为school的数据库并设置默认的字符集和排序方式
create database `school` default character set utf8mb4;
-- 切换到school数据库上下文环境
use `school`;
-- 创建学院表
create table `tb_college`
(
`col_id` int unsigned auto_increment comment '编号',
`col_name` varchar(50) not null comment '名称',
`col_intro` varchar(5000) default '' comment '介绍',
primary key (`col_id`)
) engine=innodb comment '学院表';
-- 创建学生表
create table `tb_student`
(
`stu_id` int unsigned not null comment '学号',
`stu_name` varchar(20) not null comment '姓名',
`stu_sex` boolean default 1 comment '性别',
`stu_birth` date not null comment '出生日期',
`stu_addr` varchar(255) default '' comment '籍贯',
`col_id` int unsigned not null comment '所属学院',
primary key (`stu_id`),
foreign key (`col_id`) references `tb_college` (`col_id`)
) engine=innodb comment '学生表';
-- 创建教师表
create table `tb_teacher`
(
`tea_id` int unsigned not null comment '工号',
`tea_name` varchar(20) not null comment '姓名',
`tea_title` varchar(10) default '助教' comment '职称',
`col_id` int unsigned not null comment '所属学院',
primary key (`tea_id`),
foreign key (`col_id`) references `tb_college` (`col_id`)
) engine=innodb comment '老师表';
-- 创建课程表
create table `tb_course`
(
`cou_id` int unsigned not null comment '编号',
`cou_name` varchar(50) not null comment '名称',
`cou_credit` int not null comment '学分',
`tea_id` int unsigned not null comment '授课老师',
primary key (`cou_id`),
foreign key (`tea_id`) references `tb_teacher` (`tea_id`)
) engine=innodb comment '课程表';
-- 创建选课记录表
create table `tb_record`
(
`rec_id` bigint unsigned auto_increment comment '选课记录号',
`sid` int unsigned not null comment '学号',
`cid` int unsigned not null comment '课程编号',
`sel_date` date not null comment '选课日期',
`score` decimal(4,1) comment '考试成绩',
primary key (`rec_id`),
foreign key (`sid`) references `tb_student` (`stu_id`),
foreign key (`cid`) references `tb_course` (`cou_id`),
unique (`sid`, `cid`)
) engine=innodb comment '选课记录表';
DML(数据操作语言)
use school;
-- 插入学院数据
insert into `tb_college`
(`col_name`, `col_intro`)
values
('计算机学院', '计算机学院1958年设立计算机专业，1981年建立计算机科学系，1998年设立计算机学院，2005年5月，为了进一步整合教学和科研资源，学校决定，计算机学院和软件学院行政班子合并统一运作、实行教学和学生管理独立运行的模式。 学院下设三个系：计算机科学与技术系、物联网工程系、计算金融系；两个研究所：图象图形研究所、网络空间安全研究院（2015年成立）；三个教学实验中心：计算机基础教学实验中心、IBM技术中心和计算机专业实验中心。'),
('外国语学院', '外国语学院设有7个教学单位，6个文理兼收的本科专业；拥有1个一级学科博士授予点，3个二级学科博士授予点，5个一级学科硕士学位授权点，5个二级学科硕士学位授权点，5个硕士专业授权领域，同时还有2个硕士专业学位（MTI）专业；有教职员工210余人，其中教授、副教授80余人，教师中获得中国国内外名校博士学位和正在职攻读博士学位的教师比例占专任教师的60%以上。'),
('经济管理学院', '经济学院前身是创办于1905年的经济科；已故经济学家彭迪先、张与九、蒋学模、胡寄窗、陶大镛、胡代光，以及当代学者刘诗白等曾先后在此任教或学习。');
-- 插入学生数据
insert into `tb_student`
(`stu_id`, `stu_name`, `stu_sex`, `stu_birth`, `stu_addr`, `col_id`)
values
(1001, '杨过', 1, '1990-3-4', '湖南长沙', 1),
(1002, '任我行', 1, '1992-2-2', '湖南长沙', 1),
(1033, '王语嫣', 0, '1989-12-3', '四川成都', 1),
(1572, '岳不群', 1, '1993-7-19', '陕西咸阳', 1),
(1378, '纪嫣然', 0, '1995-8-12', '四川绵阳', 1),
(1954, '林平之', 1, '1994-9-20', '福建莆田', 1),
(2035, '东方不败', 1, '1988-6-30', null, 2),
(3011, '林震南', 1, '1985-12-12', '福建莆田', 3),
(3755, '项少龙', 1, '1993-1-25', null, 3),
(3923, '杨不悔', 0, '1985-4-17', '四川成都', 3);
-- 插入老师数据
insert into `tb_teacher`
(`tea_id`, `tea_name`, `tea_title`, `col_id`)
values
(1122, '张三丰', '教授', 1),
(1133, '宋远桥', '副教授', 1),
(1144, '杨逍', '副教授', 1),
(2255, '范遥', '副教授', 2),
(3366, '韦一笑', default, 3);
-- 插入课程数据
insert into `tb_course`
(`cou_id`, `cou_name`, `cou_credit`, `tea_id`)
values
(1111, 'Python程序设计', 3, 1122),
(2222, 'Web前端开发', 2, 1122),
(3333, '操作系统', 4, 1122),
(4444, '计算机网络', 2, 1133),
(5555, '编译原理', 4, 1144),
(6666, '算法和数据结构', 3, 1144),
(7777, '经贸法语', 3, 2255),
(8888, '成本会计', 2, 3366),
(9999, '审计学', 3, 3366);
-- 插入选课数据
insert into `tb_record`
(`sid`, `cid`, `sel_date`, `score`)
values
(1001, 1111, '2017-09-01', 95),
(1001, 2222, '2017-09-01', 87.5),
(1001, 3333, '2017-09-01', 100),
(1001, 4444, '2018-09-03', null),
(1001, 6666, '2017-09-02', 100),
(1002, 1111, '2017-09-03', 65),
(1002, 5555, '2017-09-01', 42),
(1033, 1111, '2017-09-03', 92.5),
(1033, 4444, '2017-09-01', 78),
(1033, 5555, '2017-09-01', 82.5),
(1572, 1111, '2017-09-02', 78),
(1378, 1111, '2017-09-05', 82),
(1378, 7777, '2017-09-02', 65.5),
(2035, 7777, '2018-09-03', 88),
(2035, 9999, '2019-09-02', null),
(3755, 1111, '2019-09-02', null),
(3755, 8888, '2019-09-02', null),
(3755, 9999, '2017-09-01', 92);
DQL(数据查询语言)
-- 查询所有学生的所有信息
select * from tb_student;
select stu_id, stu_name, stu_sex, stu_birth, stu_addr, col_id from tb_student;
-- 查询所有课程名称及学分(投影和别名)
select cou_name as 课程名称, cou_credit as 学分 from tb_course;
-- 查询所有女学生的姓名和出生日期(筛选)
select stu_name, stu_birth from tb_student where stu_sex=0;
-- 查询所有80后学生的姓名、性别和出生日期(筛选)
select stu_name, stu_sex, stu_birth from tb_student
where stu_birth>='1980-1-1' and stu_birth<='1989-12-31';
select stu_name, stu_sex, stu_birth from tb_student
where stu_birth between '1980-1-1' and '1989-12-31';
-- 补充1：在查询时可以对列的值进行处理
select
stu_name as 姓名,
case stu_sex when 1 then '男' else '女' end as 性别,
stu_birth as 生日
from tb_student
where stu_birth between '1980-1-1' and '1989-12-31';
-- 补充2：MySQL方言(使用数据库特有的函数)
-- 例如：Oracle中做同样事情的函数叫做decode
select
stu_name as 姓名,
if(stu_sex, '男', '女') as 性别,
stu_birth as 生日
from tb_student
where stu_birth between '1980-1-1' and '1989-12-31';
-- 查询所有80后女学生的姓名和出生日期
select stu_name, stu_birth from tb_student
where stu_birth between '1980-1-1' and '1989-12-31' and stu_sex=0;
-- 查询所有的80后学生或女学生的姓名和出生日期
select stu_name, stu_birth from tb_student
where stu_birth between '1980-1-1' and '1989-12-31' or stu_sex=0;
-- 查询姓“杨”的学生姓名和性别(模糊)
-- 在SQL中通配符%可以匹配零个或任意多个字符
select stu_name, stu_sex from tb_student where stu_name like '杨%';
-- 查询姓“杨”名字两个字的学生姓名和性别(模糊)
-- 在SQL中通配符_可以刚刚好匹配一个字符
select stu_name, stu_sex from tb_student where stu_name like '杨_';
-- 查询姓“杨”名字三个字的学生姓名和性别(模糊)
select stu_name, stu_sex from tb_student where stu_name like '杨__';
-- 查询名字中有“不”字或“嫣”字的学生的姓名(模糊)
-- 提示：前面带%的模糊查询性能基本上都是非常糟糕的
select stu_name from tb_student
where stu_name like '%不%' or stu_name like '%嫣%';
update tb_student set stu_name='岳不嫣' where stu_id=1572;
-- 并集运算
select stu_name from tb_student where stu_name like '%不%'
union
select stu_name from tb_student where stu_name like '%嫣%';
select stu_name from tb_student where stu_name like '%不%'
union all
select stu_name from tb_student where stu_name like '%嫣%';
-- 正则表达式模糊查询
select stu_name, stu_sex from tb_student where stu_name regexp '^杨.{2}$';
-- 查询没有录入家庭住址的学生姓名(空值)
-- null作任何运算结果也是产生null，null相当于是条件不成立
select stu_name from tb_student where stu_addr is null;
select stu_name from tb_student where stu_addr<=>null;
-- 查询录入了家庭住址的学生姓名(空值)
select stu_name from tb_student where stu_addr is not null;
-- 查询学生选课的所有日期(去重)
select distinct sel_date from tb_record;
-- 查询学生的家庭住址(去重)
select distinct stu_addr from tb_student where stu_addr is not null;
-- 查询男学生的姓名和生日按年龄从大到小排列(排序)
-- asc - 升序（从小到大），desc - 降序（从大到小）
select stu_name, stu_birth from tb_student
where stu_sex=1 order by stu_birth asc;
select stu_name, stu_birth from tb_student
where stu_sex=1 order by stu_birth desc;
-- 查询年龄最大的学生的出生日期(聚合函数) ---> 找出最小的生日
select min(stu_birth) from tb_student;
select
min(stu_birth) as 生日,
floor(datediff(curdate(), min(stu_birth))/365) as 年龄
from tb_student;
-- 查询年龄最小的学生的出生日期(聚合函数)
select
max(stu_birth) as 生日,
floor(datediff(curdate(), max(stu_birth))/365) as 年龄
from tb_student;
-- 查询所有考试的平均成绩
-- 聚合函数在遇到null值会做忽略的处理
-- 如果做计数操作，建议使用count(*)，这样才不会漏掉空值
select avg(score) from tb_record;
select sum(score) / count(score) from tb_record;
select sum(score) / count(*) from tb_record;
-- 查询课程编号为1111的课程的平均成绩(筛选和聚合函数)
select avg(score) from tb_record where cid=1111;
-- 查询学号为1001的学生所有课程的平均分(筛选和聚合函数)
select avg(score) from tb_record where sid=1001;
select count(distinct stu_addr) from tb_student where stu_addr is not null;
-- 查询男女学生的人数(分组和聚合函数)
-- SAC（Split - Aggregate - Combine）
select
if(stu_sex, '男', '女') as 性别,
count(*) as 人数
from tb_student group by stu_sex;
-- 统计每个学院男女学生的人数
select
col_id as 学院,
if(stu_sex, '男', '女') as 性别,
count(*) as 人数
from tb_student group by col_id, stu_sex;
-- 查询每个学生的学号和平均成绩(分组和聚合函数)
select
sid as 学号,
round(avg(score),1) as 平均分
from tb_record group by sid;
-- 查询平均成绩大于等于90分的学生的学号和平均成绩
-- 分组以前的数据筛选使用where子句，分组以后的数据筛选使用having子句
select
sid as 学号,
round(avg(score),1) as 平均分
from tb_record
group by sid having 平均分>=90;
-- 查询年龄最大的学生的姓名(子查询)
-- 嵌套查询：把一个查询的结果作为另外一个查询的一部分来使用。
select stu_name from tb_student where stu_birth=(
select min(stu_birth) from tb_student
);
-- 查询年龄最大的学生姓名和年龄(子查询+运算)
select
stu_name as 姓名,
floor(datediff(curdate(), stu_birth) / 365) as 年龄
from tb_student where stu_birth=(
select min(stu_birth) from tb_student
);
-- 查询选了两门以上的课程的学生姓名(子查询/分组条件/集合运算)
select stu_name from tb_student where stu_id in (
select sid from tb_record group by sid having count(*)>2
);
-- 查询课程的名称、学分和授课老师的姓名(连接查询)
select cou_name, cou_credit, tea_name
from tb_course, tb_teacher
where tb_course.tea_id=tb_teacher.tea_id;
select cou_name, cou_credit, tea_name from tb_course t1
inner join tb_teacher t2 on t1.tea_id=t2.tea_id;
-- 查询学生姓名、课程名称以及成绩(连接查询)
select stu_name, cou_name, score
from tb_record, tb_student, tb_course
where stu_id=sid and cou_id=cid and score is not null;
select stu_name, cou_name, score from tb_student
inner join tb_record on stu_id=sid
inner join tb_course on cou_id=cid
where score is not null;
-- 查询选课学生的姓名和平均成绩(子查询和连接查询)
select stu_name, avg_score
from tb_student, (select sid, round(avg(score),1) as avg_score
from tb_record group by sid
) tb_temp where stu_id=sid;
-- 分页查询
select stu_name, cou_name, score
from tb_student natural join tb_record natural join tb_course
where score is not null
order by score desc limit 5;
select stu_name, cou_name, score
from tb_student natural join tb_record natural join tb_course
where score is not null
order by score desc limit 5 offset 5;
select stu_name, cou_name, score
from tb_student natural join tb_record natural join tb_course
where score is not null
order by score desc limit 5 offset 10; -- 表示 跳过前10个 取五个
select stu_name, cou_name, score
from tb_student natural join tb_record natural join tb_course
where score is not null
order by score desc limit 10,5; -- 表示 跳过前10个 取五个
-- 查询每个学生的姓名和选课数量(左外连接和子查询)
-- 内连接：查询左右两表满足连接条件的数据。
-- 外连接
-- 左外连接：确保左表（现在join前面的表）中的所有记录都能查出来，不满足连接条件的补充null。
-- 右外连接：确保右表（现在join后面的表）中的所有记录都能查出来，不满足连接条件的补充null。
-- 全外连接：确保左表和右表中的所有记录都能查出来，不满足连接条件的补充null。
select
stu_name as 姓名,
ifnull(total, 0) as 选课数量
from tb_student left outer join (
select sid, count(*) as total from tb_record group by sid
) tb_temp on stu_id=sid;
-- 右外连接
select t1.stu_id, stu_name, t2.stu_id, total as total
from tb_student t1 right outer join (
select stu_id, count(*) as total from tb_record
group by stu_id
) t2 on t1.stu_id=t2.stu_id;
-- MySQL不支持全外连接
-- 可以通过左外连接与右外连接求并集运算得到全外连接的结果
select t1.stu_id, stu_name, t2.stu_id, total as total
from tb_student t1 left outer join (
select stu_id, count(*) as total from tb_record
group by stu_id
) t2 on t1.stu_id=t2.stu_id
union
select t1.stu_id, stu_name, t2.stu_id, total as total
from tb_student t1 right outer join (
select stu_id, count(*) as total from tb_record
group by stu_id
) t2 on t1.stu_id=t2.stu_id;
DCL（数据控制语言）
-- 创建可以远程登录的root账号并为其指定口令
create user 'root'@'%' identified by '123456';
-- 为远程登录的root账号授权操作所有数据库所有对象的所有权限并允许其将权限再次赋予其他用户
grant all privileges on *.* to 'root'@'%' with grant option;
-- 创建名为hellokitty的用户并为其指定口令
create user 'hellokitty'@'%' identified by '123123';
-- 将对school数据库所有对象的所有操作权限授予hellokitty
grant all privileges on school.* to 'hellokitty'@'%';
-- 召回hellokitty对school数据库所有对象的insert/delete/update权限
revoke insert, delete, update on school.* from 'hellokitty'@'%';
##############################################
-- DCL 数据控制语言
-- 创建用户
create user 'guest' @'10.7.178.%' identified by 'Guest.618';
-- 删除用户
drop user 'guest' @'10.7.178.%';
-- 修改密码
alter user 'guest' @'10.7.178.%' identified by 'Hellokitty.123';
-- 授权
grant select on hrs.* to 'guest'@'10.7.178.%';
grant insert on hrs.* to 'guest'@'10.7.178.%';
-- 授权所有权限
grant all privileges on hrs.* to 'guest'@'10.7.178.%';
-- 授权所有权限,对于所有对象
grant all privileges on *.* to 'guest'@'10.7.178.%' ;
grant all privileges on *.* to 'guest'@'10.7.178.%' with grant option;
-- 招回权限
revoke insert on hrs.* from 'guest'@'10.7.178.%';
-- 让权限立即生效
flush privileges
数据库索引与性能
-- 如何了解SQL语句的性能？
-- explain 查看索引性能
-- 索引的本质就相当于是一个排好序的目录，加速查询
-- 索引通常要建在经常用户查询筛选条件的列上，这样才能有效的加速查询
-- 不要滥用索引，因为索引虽然加速了查询，但是会让插入数据的操作变得更慢
-- 对于InnoDB引擎来说，索引的底层是一个B+树，B+树是一个层次结构
-- 这种结构拥有极好磁盘I/O性能，一个4层的B+树就能应对10亿级的数据体量
-- 对于InnoDB引擎来说，主键上默认就会建索引，而且索引就是整张表的数据
-- 这种索引也称为聚集索引（只能有一个），而我们自己创建的索引都是非聚集索引
-- 1. 查看SQL执行计划
-- 2. 使用性能剖析系统
explain select * from tb_emp where eno=7800;
explain select * from tb_emp where ename='张三丰';
-- 创建索引
create index idx_emp_ename on tb_emp (ename);
-- 前缀索引
create index idx_emp_ename on tb_emp (ename(1));
-- 索引的覆盖性问题
-- 如果查询的列没有被索引覆盖到，就会产生回表现象
-- 如果不希望产生回表现象，那么我们在创建索引时，可以使用复合索引尽可能覆盖查询需要的所有的列
select eno, ename, job from tb_emp where ename='张三丰';
-- 复合索引 -- 左索引开始
drop index idx_emp_ename_job on tb_emp;
create index idx_emp_ename_job on tb_emp (ename, job);
explain select eno, ename, job from tb_emp where ename='张三丰';
explain select eno, ename, job from tb_emp where ename like '张%' order by ename;
explain select eno, ename, job from tb_emp where ename like '张%' order by sal;
explain select eno, ename, job from tb_emp
where ename='张三丰' and job='总裁';
-- 负向条件无法使用索引优化查询
explain select eno, ename, job from tb_emp where ename<>'张三丰';
explain select eno, ename, job from tb_emp where ename like '张三%';
-- 模糊查询如果左边使用通配符会导致索引失效
explain select eno, ename, job from tb_emp where ename like '%三丰';
-- 在筛选条件中如果使用了函数也会导致索引失效
explain select eno, ename, job from tb_emp where concat(ename, '先生')='张三丰先生';
-- 下面的查询无法使用复合索引
explain select eno, ename, job from tb_emp
where ename='张三丰' or job='总裁';
-- 下面的查询无法使用复合索引
explain select eno, ename, job from tb_emp where job='总裁';
-- 删除索引
drop index idx_emp_ename on tb_emp;
alter table tb_emp drop index idx_emp_ename;
数据库存储与性能
use hrs;
show variables like '%safe%';
-- （存储）过程：将一系列的SQL组装到一起编译存储起来以备调用
-- 删除存储过程
drop procedure if exists sp_upgrade_emp_sal;
-- 创建存储过程
delimiter $$
create procedure sp_upgrade_emp_sal()
begin
set sql_safe_updates=off;
update tb_emp set sal=sal+200 where dno=10;
update tb_emp set sal=sal+500 where dno=20;
update tb_emp set sal=sal+800 where dno=30;
end $$
delimiter ;
-- 调用存储过程
call sp_upgrade_emp_sal();
delimiter $$
create procedure sp_get_avg_sal(
in dept_no int,
out avg_sal decimal(6,1)
)
begin
select round(avg(sal),1) into avg_sal from tb_emp where dno=dept_no;
end $$
delimiter ;
call sp_get_avg_sal(10, @a);
select @a;
数据库视图
-- 视图
-- 给查询生成一个快照，以后可以直接通过视图获得查询结果
-- 可以通过视图将用户访问数据的权限限制到具体的列上
-- 创建视图
create view vw_emp_simple as
select eno, ename, job, dno from tb_emp;
-- 更新视图实际更新的是视图对应的原始表的数据
update vw_emp_simple set ename='胡二刀' where eno=1359;
drop user if exists 'wangdachui'@'%';
create user 'wangdachui'@'%' identified
with mysql_native_password by '654321';
-- 授权用户只能查看视图
grant select, show view on hrs.vw_emp_simple to 'wangdachui'@'%';
flush privileges;
create view vw_sal_top2 as
select ename, sal, dno from tb_emp t1 where (
select count(*) from tb_emp t2
where t1.dno=t2.dno and t2.sal>t1.sal
)<2 order by dno asc, sal desc;
select * from vw_sal_top2;
create view vw_dept_emp_count as
select dname, ifnull(total, 0) as total from tb_dept t1 left join
(select dno, count(*) as total from tb_emp group by dno) t2
on t1.dno=t2.dno;
-- 不能更新视图，因为视图的数据是通过聚合、运算等操作得到的
update vw_dept_emp_count set total=100 where dname='研发2部';
-- 删除视图
drop view vw_dept_emp_count;
数据库权限授予
use hrs;
-- DCL 数据控制语言
-- 创建用户
create user 'guest' @'10.7.178.%' identified by 'Guest.618';
-- 删除用户
drop user 'guest' @'10.7.178.%';
-- 修改密码
alter user 'guest' @'10.7.178.%' identified by 'Hellokitty.123';
-- 授权
grant select on hrs.* to 'guest'@'10.7.178.%';
grant insert on hrs.* to 'guest'@'10.7.178.%';
-- 授权所有权限
grant all privileges on hrs.* to 'guest'@'10.7.178.%';
-- 授权所有权限,对于所有对象
grant all privileges on *.* to 'guest'@'10.7.178.%' ;
grant all privileges on *.* to 'guest'@'10.7.178.%' with grant option;
-- 招回权限
revoke insert on hrs.* from 'guest'@'10.7.178.%';
-- 让权限立即生效
flush privileges
练习
school库建立
-- 如果存在名为school的数据库就删除它
drop database if exists `school`;
-- 创建名为school的数据库并设置默认的字符集和排序方式
create database `school` default character set utf8mb4;
-- 切换到school数据库上下文环境
use `school`;
-- 创建学院表
create table `tb_college`
(
`col_id` int unsigned auto_increment comment '编号',
`col_name` varchar(50) not null comment '名称',
`col_intro` varchar(500) default '' comment '介绍',
primary key (`col_id`)
) engine=innodb auto_increment=1 comment '学院表';
-- 创建学生表
create table `tb_student`
(
`stu_id` int unsigned not null comment '学号',
`stu_name` varchar(20) not null comment '姓名',
`stu_sex` boolean default 1 not null comment '性别',
`stu_birth` date not null comment '出生日期',
`stu_addr` varchar(255) default '' comment '籍贯',
`col_id` int unsigned not null comment '所属学院',
primary key (`stu_id`),
constraint `fk_student_col_id` foreign key (`col_id`) references `tb_college` (`col_id`)
) engine=innodb comment '学生表';
-- 创建教师表
create table `tb_teacher`
(
`tea_id` int unsigned not null comment '工号',
`tea_name` varchar(20) not null comment '姓名',
`tea_title` varchar(10) default '助教' comment '职称',
`col_id` int unsigned not null comment '所属学院',
primary key (`tea_id`),
constraint `fk_teacher_col_id` foreign key (`col_id`) references `tb_college` (`col_id`)
) engine=innodb comment '老师表';
-- 创建课程表
create table `tb_course`
(
`cou_id` int unsigned not null comment '编号',
`cou_name` varchar(50) not null comment '名称',
`cou_credit` int not null comment '学分',
`tea_id` int unsigned not null comment '授课老师',
primary key (`cou_id`),
constraint `fk_course_tea_id` foreign key (`tea_id`) references `tb_teacher` (`tea_id`)
) engine=innodb comment '课程表';
-- 创建选课记录表
create table `tb_record`
(
`rec_id` bigint unsigned auto_increment comment '选课记录号',
`stu_id` int unsigned not null comment '学号',
`cou_id` int unsigned not null comment '课程编号',
`sel_date` date not null comment '选课日期',
`score` decimal(4,1) comment '考试成绩',
primary key (`rec_id`),
constraint `fk_record_stu_id` foreign key (`stu_id`) references `tb_student` (`stu_id`),
constraint `fk_record_cou_id` foreign key (`cou_id`) references `tb_course` (`cou_id`),
constraint `uk_record_stu_cou` unique (`stu_id`, `cou_id`)
) engine=innodb comment '选课记录表';
-- 插入学院数据
insert into `tb_college`
(`col_name`, `col_intro`)
values
('计算机学院', '计算机学院1958年设立计算机专业，1981年建立计算机科学系，1998年设立计算机学院，2005年5月，为了进一步整合教学和科研资源，学校决定，计算机学院和软件学院行政班子合并统一运作、实行教学和学生管理独立运行的模式。 学院下设三个系：计算机科学与技术系、物联网工程系、计算金融系；两个研究所：图象图形研究所、网络空间安全研究院（2015年成立）；三个教学实验中心：计算机基础教学实验中心、IBM技术中心和计算机专业实验中心。'),
('外国语学院', '外国语学院设有7个教学单位，6个文理兼收的本科专业；拥有1个一级学科博士授予点，3个二级学科博士授予点，5个一级学科硕士学位授权点，5个二级学科硕士学位授权点，5个硕士专业授权领域，同时还有2个硕士专业学位（MTI）专业；有教职员工210余人，其中教授、副教授80余人，教师中获得中国国内外名校博士学位和正在职攻读博士学位的教师比例占专任教师的60%以上。'),
('经济管理学院', '经济学院前身是创办于1905年的经济科；已故经济学家彭迪先、张与九、蒋学模、胡寄窗、陶大镛、胡代光，以及当代学者刘诗白等曾先后在此任教或学习。');
-- 插入学生数据
insert into `tb_student`
(`stu_id`, `stu_name`, `stu_sex`, `stu_birth`, `stu_addr`, `col_id`)
values
(1001, '留一手', 1, '1990-3-4', '湖南长沙', 1),
(1002, '二麻子', 1, '1992-2-2', '台湾台北', 1),
(1033, '法外狂徒张三', 0, '1989-12-3', '北京', 1),
(1572, '四季度', 1, '1993-7-19', '陕西咸阳', 1),
(1378, '五条悟', 0, '1995-8-12', '四川成都', 1),
(1954, '小六子', 1, '1994-9-20', '福建莆田', 1),
(2035, '鬼脚七', 1, '1988-6-30', null, 2),
(3011, '老八', 1, '1985-12-12', '海南海口', 3),
(3755, '九妹', 1, '1993-1-25', null, 3),
(3923, '大十字', 0, '1985-4-17', '贵州贵阳', 3);
-- 插入老师数据
insert into `tb_teacher`
(`tea_id`, `tea_name`, `tea_title`, `col_id`)
values
(1122, '萧十一郎', '教授', 1),
(1133, '十二月', '副教授', 1),
(1144, '十三姨', '副教授', 1),
(2255, '十四松', '副教授', 2),
(3366, '桃十五', default, 3);
-- 插入课程数据
insert into `tb_course`
(`cou_id`, `cou_name`, `cou_credit`, `tea_id`)
values
(1111, 'Python程序设计', 3, 1122),
(2222, '信号与图像处理', 5, 1122),
(3333, '高等数学', 5, 1122),
(4444, '计算机网络', 3, 1133),
(5555, '离散数学', 3, 1144),
(6666, '数据结构', 5, 1144),
(7777, '数据挖掘', 3, 2255),
(8888, '大数据技术原理与应用', 2, 3366),
(9999, '人工智能导论', 3, 3366);
-- 插入选课数据
insert into `tb_record`
(`stu_id`, `cou_id`, `sel_date`, `score`)
values
(1001, 1111, '2017-09-01', 95),
(1001, 2222, '2019-09-01', 87.5),
(1001, 3333, '2017-09-01', 100),
(1001, 4444, '2018-09-03', null),
(1001, 6666, '2018-09-02', 100),
(1002, 1111, '2017-09-03', 65),
(1002, 5555, '2018-09-01', 42),
(1033, 1111, '2017-09-03', 92.5),
(1033, 4444, '2019-09-01', 78),
(1033, 5555, '2017-09-01', 82.5),
(1572, 1111, '2020-09-02', 78),
(1378, 1111, '2020-09-05', 82),
(1378, 7777, '2020-09-02', 65.5),
(2035, 7777, '2018-09-03', 88),
(2035, 9999, '2019-09-02', null),
(3755, 1111, '2019-09-02', null),
(3755, 8888, '2019-09-02', null),
(3755, 9999, '2017-09-01', 92);
开始查询
use school;
-- 查询所有学生的所有信息
select * from tb_student;
-- 查询学生的学号、姓名和家庭住址(投影)
select stu_id, stu_name, stu_addr from tb_student;
-- 查询所有课程的名称及学分(投影和别名) ---> alias
select cou_name as 课程名称, cou_credit as 学分 from tb_course;
-- 查询所有女学生的姓名和出生日期(筛选)
select stu_name, stu_birth from tb_student where stu_sex=0;
-- 查询所有80后学生的姓名、性别和出生日期(筛选)
select stu_name, stu_sex, stu_birth
from tb_student
where stu_birth>='1980-1-1' and stu_birth<='1989-12-31';
select stu_name, stu_sex, stu_birth
from tb_student
where stu_birth between '1980-1-1' and '1989-12-31';
-- 标准SQL的做法
select
stu_name as 姓名,
case stu_sex when 1 then '男' else '女' end as 性别,
stu_birth as 生日
from tb_student
where stu_birth between '1980-1-1' and '1989-12-31';
-- MySQL方言 if() 函数 ---> Oracle方言 decode() 函数
select
stu_name as 姓名,
if(stu_sex, '男', '女') as 性别,
stu_birth as 生日
from tb_student
where stu_birth between '1980-1-1' and '1989-12-31';
-- 查询姓“杨”的学生姓名和性别(模糊)
-- 通配符（wildcard）---> % ---> 匹配零个或任意多个字符
select stu_name, stu_sex from tb_student where stu_name like '杨%';
-- 查询姓“杨”名字两个字的学生姓名和性别(模糊)
-- 通配符（wildcard）---> _ ---> 精确匹配一个字符
select stu_name, stu_sex from tb_student where stu_name like '杨_';
-- 查询姓“杨”名字三个字的学生姓名和性别(模糊)
select stu_name, stu_sex from tb_student where stu_name like '杨__';
-- 查询名字中有“不”字或“嫣”字的学生的姓名(模糊)
select stu_name from tb_student
where stu_name like '%不%' or stu_name like '%嫣%';
select stu_name from tb_student where stu_name like '%嫣%'
union
select stu_name from tb_student where stu_name like '%不%';
-- 基于正则表达式的模糊查询
select stu_name from tb_student where stu_name regexp '[杨林].{2}';
-- 查询没有录入家庭住址的学生姓名(空值)
select stu_name from tb_student
where stu_addr is null or stu_add='';
select stu_name from tb_student
where stu_addr<=>null or stu_addr='';
-- 查询录入了家庭住址的学生姓名(空值)
select stu_name from tb_student
where stu_addr is not null and stu_addr<>'';
-- 查询学生选课的所有日期(去重)
select distinct sel_date from tb_record;
-- 查询学生的家庭住址(去重)
select distinct stu_addr from tb_student
where stu_addr is not null;
-- 查询男学生的姓名和生日按年龄从大到小排列(排序)
-- 升序（从小到大）---> ascending
select stu_name, stu_birth from tb_student
where stu_sex=1
order by stu_birth asc;
-- curdate ---> 获取当前日期
-- datediff ---> 计算时间差（以天为单位）
-- floor / ceil ---> 向下/上取整
select
stu_name,
stu_birth,
floor(datediff(curdate(), stu_birth)/365) as stu_age
from tb_student
where stu_sex=1
order by stu_age desc;
-- 降序（从大到小）---> descending
select stu_name, stu_birth from tb_student
where stu_sex=1
order by stu_birth desc;
-- 查询年龄最大的学生的出生日期(聚合函数)
select min(stu_birth) from tb_student;
-- 查询年龄最小的学生的出生日期(聚合函数)
select max(stu_birth) from tb_student;
-- 查询编号为1111的课程考试成绩的最高分
select max(score) from tb_record where cou_id=1111;
-- 查询学号为1001的学生考试成绩的最低分
select min(score) from tb_record where stu_id=1001;
-- 查询学号为1001的学生考试成绩的平均分
select avg(score) from tb_record where stu_id=1001;
select sum(score) / count(score) from tb_record where stu_id=1001;
-- 查询学号为1001的学生考试成绩的平均分，如果有null值，null值算0分
select sum(score) / count(*) from tb_record where stu_id=1001;
select avg(ifnull(score, 0)) from tb_record where stu_id=1001;
-- ifnull ---> 如果遇到null（空值），将其替换为指定的值
-- 查询学号为1001的学生考试成绩的标准差
select std(score) from tb_record where stu_id=1001;
-- 查询男女学生的人数(分组和聚合函数)
select
if(stu_sex, '男', '女') as 性别,
count(*) as 人数
from tb_student group by stu_sex;
-- 查询每个学院男女学生人数
select
col_id as 学院编号,
if(stu_sex, '男', '女') as 性别,
count(*) as 人数
from tb_student group by col_id, stu_sex;
-- 查询每个学生的学号和平均成绩(分组和聚合函数)
select
stu_id as 学号,
round(avg(score), 2) as 平均分
from tb_record group by stu_id;
-- 查询平均成绩大于等于90分的学生的学号和平均成绩
select
stu_id as 学号,
round(avg(score), 2) as 平均分
from tb_record
group by stu_id
having 平均分>=90;
-- 查询1111、2222、3333三门课程平均成绩大于等于90分的学生的学号和平均成绩
select
stu_id as 学号,
round(avg(score), 2) as 平均分
from tb_record
where cou_id in (1111, 2222, 3333)
group by stu_id
having 平均分>=90;
-- 分组以前的数据筛选使用where子句，分组以后的数据筛选使用having子句
-- 查询年龄最大的学生的姓名(子查询)
set @a=(select min(stu_birth) from tb_student);
select @a:=(select min(stu_birth) from tb_student);
select @a;
select stu_name from tb_student where stu_birth=@a;
-- 嵌套查询：把一个select的结果作为另一个select的一部分来使用
-- 嵌套查询通常也称之为子查询，在查询语句中有两个或多个select
select stu_name from tb_student
where stu_birth=(
select min(stu_birth) from tb_student
);
-- 查询选了两门以上的课程的学生姓名(子查询/分组条件/集合运算)
select stu_name from tb_student
where stu_id in (
select stu_id from tb_record
group by stu_id having count(*)>2
);
-- 查询学生的姓名、生日和所在学院名称
select stu_name, stu_birth, col_name
from tb_student, tb_college
where tb_student.col_id=tb_college.col_id;
select stu_name, stu_birth, col_name
from tb_student t1 inner join tb_college t2
on t1.col_id=t2.col_id;
select stu_name, stu_birth, col_name
from tb_student natural join tb_college;
-- 查询学生姓名、课程名称以及成绩(连接查询/联结查询)
select t2.stu_id, stu_name, t3.cou_id, cou_name, score
from tb_record t1, tb_student t2, tb_course t3
where
t1.stu_id=t2.stu_id and
t1.cou_id=t3.cou_id and
score is not null;
select stu_name, cou_name, score
from tb_student t1 inner join tb_record t2
on t1.stu_id=t2.stu_id inner join tb_course t3
on t2.cou_id=t3.cou_id where score is not null;
select stu_name, cou_name, score
from tb_student natural join tb_record natural join tb_course
where score is not null;
-- 分页查询
select stu_name, cou_name, score
from tb_student natural join tb_record natural join tb_course
where score is not null
order by score desc limit 5;
select stu_name, cou_name, score
from tb_student natural join tb_record natural join tb_course
where score is not null
order by score desc limit 5 offset 5;
select stu_name, cou_name, score
from tb_student natural join tb_record natural join tb_course
where score is not null
order by score desc limit 5 offset 10; -- 表示 跳过前10个 取五个
select stu_name, cou_name, score
from tb_student natural join tb_record natural join tb_course
where score is not null
order by score desc limit 10,5; -- 表示 跳过前10个 取五个
-- alter table tb_record change column stu_id sid int unsigned not null;
-- alter table tb_record change column cou_id cid int unsigned not null;
-- 查询选课学生的姓名和平均成绩(子查询和连接查询)
select stu_name, avg_score
from
tb_student t1,
(
select stu_id, round(avg(score),1) as avg_score
from tb_record group by stu_id
) t2
where t1.stu_id=t2.stu_id;
-- 查询学生的姓名和选课的数量
select stu_name, total
from
tb_student t1,
(
select stu_id, count(*) as total from tb_record
group by stu_id
) t2
where t1.stu_id=t2.stu_id;
-- 查询每个学生的姓名和选课数量(左外连接和子查询)
-- 内连接：查询左右两表满足连接条件的数据。
-- 外连接
-- 左外连接：确保左表（现在join前面的表）中的所有记录都能查出来，不满足连接条件的补充null。
-- 右外连接：确保右表（现在join后面的表）中的所有记录都能查出来，不满足连接条件的补充null。
-- 全外连接：确保左表和右表中的所有记录都能查出来，不满足连接条件的补充null。
-- 左外连接
select stu_name, ifnull(total, 0) as total
from tb_student t1 left outer join (
select stu_id, count(*) as total from tb_record
group by stu_id
) t2 on t1.stu_id=t2.stu_id;
-- 删除tb_record表的外键约束
alter table tb_record drop foreign key fk_record_stu_id;
alter table tb_record drop foreign key fk_record_cou_id;
-- 给tb_record表加两条记录，学号5566在学生表没有对应的记录
insert into tb_record
values
(default, 5566, 1111, '2019-09-02', 80),
(default, 5566, 2222, '2019-09-02', 70);
-- 右外连接
select t1.stu_id, stu_name, t2.stu_id, total as total
from tb_student t1 right outer join (
select stu_id, count(*) as total from tb_record
group by stu_id
) t2 on t1.stu_id=t2.stu_id;
-- MySQL不支持全外连接
-- 可以通过左外连接与右外连接求并集运算得到全外连接的结果
select t1.stu_id, stu_name, t2.stu_id, total as total
from tb_student t1 left outer join (
select stu_id, count(*) as total from tb_record
group by stu_id
) t2 on t1.stu_id=t2.stu_id
union
select t1.stu_id, stu_name, t2.stu_id, total as total
from tb_student t1 right outer join (
select stu_id, count(*) as total from tb_record
group by stu_id
) t2 on t1.stu_id=t2.stu_id;
作业
drop database if exists hrs;
create database hrs default charset utf8mb4;
use hrs;
create table tb_dept
(
dno int not null comment '编号',
dname varchar(10) not null comment '名称',
dloc varchar(20) not null comment '所在地',
primary key (dno)
);
insert into tb_dept values
(10, '会计部', '北京'),
(20, '研发部', '成都'),
(30, '销售部', '重庆'),
(40, '运维部', '深圳');
create table tb_emp
(
eno int not null comment '员工编号',
ename varchar(20) not null comment '员工姓名',
job varchar(20) not null comment '员工职位',
mgr int comment '主管编号',
sal int not null comment '员工月薪',
comm int comment '每月补贴',
dno int comment '所在部门编号',
primary key (eno)
);
-- alter table tb_emp add constraint pk_emp_eno primary key (eno);
-- alter table tb_emp add constraint uk_emp_ename unique (ename);
-- alter table tb_emp add constraint fk_emp_mgr foreign key (mgr) references tb_emp (eno);
-- alter table tb_emp add constraint fk_emp_dno foreign key (dno) references tb_dept (dno);
insert into tb_emp values
(7800, '张三丰', '总裁', null, 9000, 1200, 20),
(2056, '乔峰', '分析师', 7800, 5000, 1500, 20),
(3088, '李莫愁', '设计师', 2056, 3500, 800, 20),
(3211, '张无忌', '程序员', 2056, 3200, null, 20),
(3233, '丘处机', '程序员', 2056, 3400, null, 20),
(3251, '张翠山', '程序员', 2056, 4000, null, 20),
(5566, '宋远桥', '会计师', 7800, 4000, 1000, 10),
(5234, '郭靖', '出纳', 5566, 2000, null, 10),
(3344, '黄蓉', '销售主管', 7800, 3000, 800, 30),
(1359, '胡一刀', '销售员', 3344, 1800, 200, 30),
(4466, '苗人凤', '销售员', 3344, 2500, null, 30),
(3244, '欧阳锋', '程序员', 3088, 3200, null, 20),
(3577, '杨过', '会计', 5566, 2200, null, 10),
(3588, '朱九真', '会计', 5566, 2500, null, 10);
-- 查询月薪最高的员工姓名和月薪
-- 查询员工的姓名和年薪((月薪+补贴)*13)
-- 查询有员工的部门的编号和人数
-- 查询所有部门的名称和人数
-- 查询月薪最高的员工(Boss除外)的姓名和月薪
-- 查询月薪超过平均月薪的员工的姓名和月薪
-- 查询月薪超过其所在部门平均月薪的员工的姓名、部门编号和月薪
-- 查询部门中月薪最高的人姓名、月薪和所在部门名称
-- 查询主管的姓名和职位
-- 查询月薪排名4~6名的员工排名、姓名和月薪
\################################
use hrs;
-- 查询月薪最高的员工姓名和月薪
select ename, sal from tb_emp where sal=(
select max(sal) from tb_emp
);
select ename, sal from tb_emp t1 where not exists (
select 'x' from tb_emp t2 where t2.sal>t1.sal
);
select ename, sal from tb_emp t1 where (
select count(*) from tb_emp t2 where t2.sal>t1.sal
)=0;
-- 查询月薪前2名的员工的姓名和月薪
select ename, sal from tb_emp t1 where (
select count(*) from tb_emp t2 where t2.sal>t1.sal
)<2;
-- 查询员工的姓名和年薪((月薪+补贴)*13)
select ename, (sal+ifnull(comm, 0))*13 as ann_sal from tb_emp;
-- 查询有员工的部门的编号和人数
select dno, count(*) as total from tb_emp group by dno;
-- 查询所有部门的名称和人数
select dname, ifnull(total, 0) as total from tb_dept t1 left join
(select dno, count(*) as total from tb_emp group by dno) t2
on t1.dno=t2.dno;
-- 查询月薪最高的员工(Boss除外)的姓名和月薪
select ename, sal from tb_emp where sal=(
select max(sal) from tb_emp where mgr is not null
);
-- 查询月薪超过平均月薪的员工的姓名和月薪
select ename, sal from tb_emp where sal>(
select avg(sal) from tb_emp
);
-- 查询月薪超过其所在部门平均月薪的员工的姓名、部门编号和月薪
select ename, t1.dno, sal from tb_emp t1 inner join
(select dno, avg(sal) as avg_sal from tb_emp group by dno) t2
on t1.dno=t2.dno and sal>avg_sal;
-- 查询部门中月薪最高的人姓名、月薪和所在部门名称
select ename, sal, dname
from
tb_emp t1, tb_dept t2,
(select dno, max(sal) as max_sal from tb_emp group by dno) t3
where t1.dno=t3.dno and sal=max_sal and t3.dno=t2.dno;
-- 查询主管的姓名和职位
select ename, job from tb_emp where eno in (
select distinct mgr from tb_emp where mgr is not null
);
-- 建议：使用 exists / not exists 替代掉 distinct 和 in / not in 运算
select ename, job from tb_emp t1 where exists (
select 'x' from tb_emp t2 where t1.eno=t2.mgr
);
-- 查询月薪排名4~6名的员工排名、姓名和月薪
select rank_num, ename, sal from
(
select @a:=@a+1 as rank_num, ename, sal
from tb_emp, (select @a:=0) t1
order by sal desc
) temp where rank_num between 4 and 6;
-- 窗口函数 ---> （离线）分析函数
-- row_number() 1 2 3 4 5 6 7 8 9 10 11
-- rank() 1 1 1 4 5 6 7 7 7 10 11
-- dense_rank() 1 1 2 3 4 4 4 5
-- 1. 查询排名问题
-- 2. Top-N查询
select rank_num, ename, sal
from (
select
dense_rank() over (order by sal desc) as rank_num,
ename,
sal
from tb_emp
) temp where rank_num between 4 and 6;
-- 查询每个部门月薪前2名的员工姓名、月薪和部门编号
select ename, sal, dno from tb_emp t1 where (
select count(*) from tb_emp t2
where t1.dno=t2.dno and t2.sal>t1.sal
)<2 order by dno asc, sal desc;
-- 使用窗口函数完成上面的查询
select ename, sal, dno
from (select
rank() over (partition by dno order by sal desc) as rank_num,
ename, sal, dno
from tb_emp) temp where rank_num<=2;
窗口函数
-- 窗口函数 ---> （离线）分析函数
-- row_number() 1 2 3 4 5 6 7 8 9 10 11
-- rank() 1 1 1 4 5 6 7 7 7 10 11
-- dense_rank() 1 1 2 3 4 4 4 5
-- 1. 查询排名问题
-- 2. Top-N查询
select rank_num, ename, sal
from (
select
dense_rank() over (order by sal desc) as rank_num,
ename,
sal
from tb_emp
) temp where rank_num between 4 and 6;
-- 查询每个部门月薪前2名的员工姓名、月薪和部门编号
select ename, sal, dno from tb_emp t1 where (
select count(*) from tb_emp t2
where t1.dno=t2.dno and t2.sal>t1.sal
)<2 order by dno asc, sal desc;
-- 使用窗口函数完成上面的查询
select ename, sal, dno
from (select
rank() over (partition by dno order by sal desc) as rank_num,
ename, sal, dno
from tb_emp) temp where rank_num<=2;