mysql练习1-10题

Ref

https://zhuanlan.zhihu.com/p/50662216

https://www.cnblogs.com/yuanyuan2017/

00 建表和添加数据

-- 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);

01课程编号为“01”的课程比“02”的课程成绩高的所有学生的学号。

很明显，需要查询的是分数score表

select s_id from score where c_id='01'

select s_id from score where c_id='02'

-- 既选了01 又选了02课的同学 我的写法

select tb1.s_id sid from

(select * from score where c_id='01') tb1,

(select * from score where c_id='02') tb2

where tb1.s_id = tb2.s_id and tb1.s_score > tb2.s_score;

-- 看了参考答案后背着写

将一张表当作两张表使用

select a.s_id from score a join score b on a.s_id=b.s_id and a.s_score > b.s_score

where a.c_id='01' and b.c_id='02' ;

-- sql99标准的表连接查询，改造最初的写法

select tb1.s_id from (select * from score where c_id='01') tb1

join (select * from score where c_id='02') tb2

on tb1.s_id=tb2.s_id

where tb1.s_score > tb2.s_score;

02查询平均成绩大于60分的学生的学号和平均成绩

查询score表，单表查询

select s_id, avg(s_score) avgScore from score GROUP BY s_id having avgScore > 60;

03 查询所有学生的学号、姓名、选课数、总成绩

两表联合查询，用student表左连接score表，可以查询出student表的所有信息。

select stu.s_id, stu.s_name,count(score.s_id), sum(score.s_score)

from student stu

left join score

on stu.s_id = score.s_id

group by stu.s_id

注意是左连接，否则会丢失数据——王菊同学一门课也没选，但是题目要求查询所有的同学。

04 查询姓“张”的老师的个数

select  teacher.*, count(*) from teacher where t_name like '张%'

05 查询没学过“张三”老师课的学生的学号、姓名(重点)

-- 最初的思路，下面的sql是查询张三老师的所有学生id

select s.s_id from student s join score

on s.s_id = score.s_id join course c

on score.c_id = c.c_id join teacher t

on c.t_id=t.t_id

where t.t_name='张三'

-- 我的做法，稍微罗嗦了点

select s_id, s_name from student where s_id not in(

select s.s_id from student s join score

on s.s_id = score.s_id join course c

on score.c_id = c.c_id join teacher t

on c.t_id=t.t_id

where t.t_name='张三'

)

-- 参考答案

select s_id,s_name from student where s_id not in (

select s_id from score join course c

on score.c_id = c.c_id join teacher t

on c.t_id=t.t_id

where t.t_name='张三'

)

06 查询学过“张三”老师所教的所有课的同学的学号、姓名

这道题是上道题的一个子集。注意灵活使用“in”关键词

select s.s_id,s.s_name from student s join score

on s.s_id = score.s_id join course c

on score.c_id = c.c_id join teacher t

on c.t_id=t.t_id

where t.t_name='张三'

参考答案

select s_id, s_name

from Student

where s_id in

(select s_id from Score join Course on Score.c_id = Course.c_id

join Teacher on Course.t_id = Teacher.t_id

where t_name = '张三');

07 查询学过编号为“01”的课程并且也学过编号为“02”的课程的学生的学号、姓名

查询course表、score表、student表

学过01课的sid

select s_id from score where c_id='01'

学过02课的sid

select s_id from score where c_id='02'

既学了课01有学了课02的sid

select t1.s_id from

(select * from score where c_id='01') t1

join

(select * from score where c_id='02') t2

on t1.s_id=t2.s_id

完整sql

select s_id, s_name from student where s_id in(

select t1.s_id from

(select * from score where c_id='01') t1

join

(select * from score where c_id='02') t2

on t1.s_id=t2.s_id

)

另一种写法

将一张表当两张表用

select s_id, s_name from student where s_id in(

select a.s_id from score a join score b

on a.c_id='01' and b.c_id='02'

where a.s_id=b.s_id

)

08 查询课程编号为“02”的总成绩

select sum(s_score) from score where c_id='02'

select sum(s_score) from (select s_score from score where c_id='02')b

09 查询所有课程成绩小于60分的学生的学号、姓名

这条sql会缺失数据，因为没有成绩的学生被排除在外了，不知道怎么解决

select * from student left join score

on student.s_id=score.s_id

group by student.s_id

having s_score < 60;

换一种思路

select * from student where s_id not in(

select s_id from score where s_score >=60

)

10 查询没有学全所有课的学生的学号、姓名

“没有学全课”意思是说总共有n门课，某同学学习的课程数小于n

查询“学全”了课的同学，再取反

select s_id from score

group by s_id

having count(c_id)=(select count(*) from course)

需要注意，没选课的同学也要被包含进去

select s_id, s_name from student where s_id not in (

select s_id from score

group by s_id

having count(c_id)=(select count(*) from course)

)

写不动了=。=晚点再来。