mysql 语法练习

练习地址：
本文用于学习mysql语法，转载于以下地址https://blog.csdn.net/fashion2014/article/details/78826299/

---

--建表

--学生表

CREATE TABLE Student(
s_id VARCHAR(20),
s_name VARCHAR(20) NOT NULL DEFAULT '',
s_birth VARCHAR(20) NOT NULL DEFAULT '',
s_sex VARCHAR(10) NOT NULL DEFAULT '',
PRIMARY KEY(s_id)
);

--课程表

CREATE TABLE Course(
c_id VARCHAR(20),
c_name VARCHAR(20) NOT NULL DEFAULT '',
t_id VARCHAR(20) NOT NULL,
PRIMARY KEY(c_id)
);

--教师表

CREATE TABLE Teacher(
t_id VARCHAR(20),
t_name VARCHAR(20) NOT NULL DEFAULT '',
PRIMARY KEY(t_id)
);

--成绩表

CREATE TABLE Score(
s_id VARCHAR(20),
c_id VARCHAR(20),
s_score INT(3),
PRIMARY KEY(s_id,c_id)
);

--插入学生表测试数据

insert into Student values('01' , '赵雷' , '1990-01-01' , '男');
insert into Student values('02' , '钱电' , '1990-12-21' , '男');
insert into Student values('03' , '孙风' , '1990-05-20' , '男');
insert into Student values('04' , '李云' , '1990-08-06' , '男');
insert into Student values('05' , '周梅' , '1991-12-01' , '女');
insert into Student values('06' , '吴兰' , '1992-03-01' , '女');
insert into Student values('07' , '郑竹' , '1989-07-01' , '女');
insert into Student values('08' , '王菊' , '1990-01-20' , '女');

--课程表测试数据

insert into Course values('01' , '语文' , '02');
insert into Course values('02' , '数学' , '01');
insert into Course values('03' , '英语' , '03');

--教师表测试数据

insert into Teacher values('01' , '张三');
insert into Teacher values('02' , '李四');
insert into Teacher values('03' , '王五');

--成绩表测试数据

insert into Score values('01' , '01' , 80);
insert into Score values('01' , '02' , 90);
insert into Score values('01' , '03' , 99);
insert into Score values('02' , '01' , 70);
insert into Score values('02' , '02' , 60);
insert into Score values('02' , '03' , 80);
insert into Score values('03' , '01' , 80);
insert into Score values('03' , '02' , 80);
insert into Score values('03' , '03' , 80);
insert into Score values('04' , '01' , 50);
insert into Score values('04' , '02' , 30);
insert into Score values('04' , '03' , 20);
insert into Score values('05' , '01' , 76);
insert into Score values('05' , '02' , 87);
insert into Score values('06' , '01' , 31);
insert into Score values('06' , '03' , 34);
insert into Score values('07' , '02' , 89);
insert into Score values('07' , '03' , 98);

查询01课程分数比02课程分数高的学生及学生信息

select s.s_id ,s.s_score as 01_course from Score as s where s.c_id = 01;
select s.s_id ,s.s_score as 02_course from Score as s where s.c_id = 02;

select D.*,E.01_course,E.02_course from Student  AS D  INNER JOIN (
select  * from (select A.s_id,A.01_course,B.02_course from (select s.s_id ,s.s_score as 01_course from Score as s where s.c_id = 01 ) as A  inner join
  (select s.s_id ,s.s_score as 02_course from Score as s where s.c_id = 02)  as B  on  A.s_id = B.s_id) as C where C.01_course >= C.02_course
                                                                  ) AS E  ON D.s_id =E.s_id;

select  a.* ,b.c_id,b.s_score ,c.c_id, c.s_score from Student a inner join Score as b on a.s_id = b.s_id and b.c_id = 01
  left join Score as c  on a.s_id = c.s_id and c.c_id = 02   where b.s_score > c.s_score

 select a.* ,b.s_score as 01_score,c.s_score as 02_score from
 student a
 join score b on a.s_id=b.s_id and b.c_id='01'
left join score c on a.s_id=c.s_id and c.c_id='02' or c.c_id = NULL where b.s_score>c.s_score

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  a.*,b.s_score , c.s_score from Student as a inner join Score as b on a.s_id = b.s_id and b.c_id = 01
left join  Score as c  on a.s_id = c.s_id and c.c_id = 02 where b.s_score <= c.s_score;

-- 3、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 b.s_id , avg(b.s_score) as avg_ from Score as b  group by b.s_id having  avg_ > 60;

select A.* ,B.avg_ from Student as A inner join ( select b.s_id , avg(b.s_score) as avg_ from Score as b  group by b.s_id having  avg_ > 60) as B on A.s_id = b.s_id;

SELECT A.s_id ,A.s_name, ROUND(AVG(B.s_score),2) FROM Student A JOIN Score B ON A.s_id = B.s_id GROUP BY A.s_id
HAVING  ROUND(AVG(B.s_score),2) >= 60;

-- 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
-- (包括有成绩的和无成绩的)

SELECT A.s_id ,A.s_name, ROUND(AVG(B.s_score),2) FROM Student A JOIN Score B ON A.s_id = B.s_id GROUP BY A.s_id
HAVING  ROUND(AVG(B.s_score),2) <= 60;



select b.s_id,b.s_name,ROUND(AVG(a.s_score),2) as avg_score from
    student b
    left join score a on b.s_id = a.s_id
    GROUP BY b.s_id,b.s_name HAVING ROUND(AVG(a.s_score),2)<60
    union
select a.s_id,a.s_name,0 as avg_score from
    student a
    where a.s_id not in (
                select distinct s_id from score);


SELECT A.s_id ,coalesce( AVG(B.s_score), 0) FROM Student A LEFT JOIN  Score B ON A.s_id = B.s_id GROUP BY A.s_id