Mysql练习题_自己做+答案+自己的理解

原文链接：https://blog.csdn.net/qq_41936662/article/details/80393172
最近学mysql，就跟着他是的做。这个文章原创有答案和题，你们可以去看，我慢慢做慢慢改。

表名和字段：
–1.学生表
Student(s_id,s_name,s_birth,s_sex) –学生编号,学生姓名, 出生年月,学生性别
–2.课程表
Course(c_id,c_name,t_id) – –课程编号, 课程名称, 教师编号
–3.教师表
Teacher(t_id,t_name) –教师编号,教师姓名
–4.成绩表
Score(s_id,c_id,s_score) –学生编号,课程编号,分数

--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"课程比"02"课程成绩高的学生的信息及课程分数

SELECT s.*,s1.s_score s01,s2.s_score s02
FROM student s
JOIN score s1 on s.s_id = s1.s_id and s1.c_id = '01'//取交集
LEFT JOIN score s2 on s.s_id = s2.s_id and s2.c_id = '02'// 取左边全集+交集
WHERE s1.s_score >IFNULL(s2.s_score,0);//把空的值设置为0

查询"01"课程比"02"课程成绩低的学生的信息及课程分数

SELECT s.*,s1.s_score s01,s2.s_score s02
FROM student s
LEFT join score s1 on s.s_id=s1.s_id and s1.c_id = '01'
join score s2 on s.s_id = s2.s_id and s2.c_id = '02'
where IFNULL(s1.s_score,0)<s2.s_score;

当要查01课程比02课程小的时候，01课程可以为空，所以用left join。

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 s.s_id,s.s_name ,ROUND(AVG(IFNULL(s1.s_score,0)),2) a
FROM student s LEFT JOIN score s1 on s.s_id = s1.s_id
GROUP BY s.s_id
HAVING ROUND(AVG(IFNULL(s1.s_score,0)),2)>=60;

由于可能出现null，而avg函数算的时候不会把null算入平局值，所以需要把null转为0

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 s.s_id,s.s_name ,ROUND(AVG(IFNULL(s1.s_score,0)),2) a
FROM student s LEFT JOIN score s1 on s.s_id = s1.s_id
GROUP BY s.s_id
HAVING ROUND(AVG(IFNULL(s1.s_score,0)),2)<=60;

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 s.s_id,s.s_name,COUNT(s1.s_id),AVG(s1.s_score)
FROM student s LEFT join score s1 on s.s_id = s1.s_id
GROUP BY s.s_id;

6、查询"李"姓老师的数量

SELECT COUNT(t.t_id)
from teacher t 
WHERE t.t_name like "李%"

7、查询学过"张三"老师授课的同学的信息

select * 
from student s
WHERE s.s_id in (SELECT s1.s_id from score s1 WHERE s1.c_id = (select t.t_id from teacher t WHERE t.t_name = "张三"));

8、查询没学过"张三"老师授课的同学的信息

select * 
from student s
WHERE s.s_id not in (SELECT s1.s_id from score s1 WHERE s1.c_id = (select t.t_id from teacher t WHERE t.t_name = "张三"));

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 a.* from 
    student a,score b,score c 
    where a.s_id = b.s_id  and a.s_id = c.s_id and b.c_id='01' and c.c_id='02';

10.查询没有学全所有课程的同学的信息

select a.*,COUNT(s.s_id) from student a
left join score s on a.s_id = s.s_id
GROUP BY a.s_id 
HAVING COUNT(s.s_id)<(SELECT count(*) from teacher)

11、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

select a.* 
from student a 
WHERE a.s_id 
in (select distinct a.s_id from score a where a.c_id in(select a.c_id from score a where a.s_id='01'))//distinct除重

Mysql练习题_自己做+答案+自己的理解

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