Mysql Sql 语句练习题 （50道）

use stu_db;
show tables;
select * from student;

-- 表信息
# student: sid, sname, sage, sgender
# teacher: tid, tname
# course: cid, cname, tid
# sc: sid, cid, score

-- 1、查询"01"课程比"02"课程成绩高的学生的信息及课程分数 
select st.*, sc1.score as '01', sc2.score as '02'
from student as st
left join sc as sc1 on st.sid = sc1.sid and sc1.cid = '01'
left join sc as sc2 on st.sid = sc2.sid and sc2.cid = '02'
where sc1.score > sc2.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 st.*, sc1.score as '01', sc2.score as '02'
from student as st
left join sc as sc1 on st.sid = sc1.sid and sc1.cid = '01'
left join sc as sc2 on st.sid = sc2.sid and sc2.cid = '02'
where sc1.score < sc2.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 student.sid, sname, avg(score) from student
left join sc on student.sid = sc.cid
group by sid
having avg(score) >= 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 student.sid, sname, (case when ROUND(avg(sc.score),2) is null then 0 else ROUND(avg(sc.score)) end ) as cjScore from student
join sc on student.sid = sc.cid
group by sid
having avg(score) >= 60 or avg(sc.score) is NULL;

-- 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 st.sid, st.sname, count(cid), avg(score)
from student as st
left join sc on sc.sid = st.sid 
group by st.sid;

-- 6、查询"李"姓老师的数量
select count(*) from teacher
where tname like'李%';

-- 7、查询学过"张三"老师授课的同学的信息
select * from student
left join sc on sc.sid = st.sid
left join course on sc.cid = course.cid
left join teacher on course.tid = teacher.tid
where teacher.tname = '张三';

-- 8、查询没学过"张三"老师授课的同学的信息 
select * from student where sid not in(
	select st.sid from student as st
	left join sc on sc.sid = st.sid
	left join course on sc.cid = course.cid
	left join teacher on course.tid = teacher.tid
	where teacher.tname = '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 st.* from student st 
inner join sc on sc.sid = st.sid and sc.cid="01"
where st.sid in (
	select st2.sid from student st2 
	inner join sc as sc2 on sc2.sid = st2.sid and sc2.cid="02"
);

SELECT st.*
FROM student as st
INNER JOIN sc sc ON sc.sid = st.sid
GROUP BY st.sid
HAVING SUM(IF(sc.cid = '01' OR sc.cid = '02' , 1, 0))>1;
# IF(expr1,expr2,expr3)  
# 如果 expr1 是TRUE (expr1 <> 0 and expr1 <> NULL)，则 IF()的返回值为expr2; 
# 否则返回值则为 expr3。IF() 的返回值为数字值或字符串值，具体情况视其所在语境而定

-- 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
select st.* from student as st
join sc on st.sid = sc.sid and sc.cid = '01'
where st.sid not in(
	select st2.sid from student as st2
	join sc as sc2 on st2.sid = sc2.sid and sc2.cid = '02'
);

-- 11、查询没有学全所有课程的同学的信息
select st.* from student as st
left join sc on st.sid = sc.sid
group by sid
having count(cid) < (select count(cid) from course);

-- 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
select distinct student.* from student
left join sc on student.sid = sc.sid
where cid in(
	select cid from sc 
	left join student as st on st.sid = sc.sid
	where st.sid = '01'
);

-- 13、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
select student.*, group_concat(cid) as '所选课程' from student
join sc using(sid)
group by sid
having group_concat(cid order by cid)  = (
	select group_concat(cid order by cid) from sc
	join student using(sid)
	where sid = '01'
);
# group_concat完整语法如下
# group_concat([DISTINCT] 要连接的字段 [Order BY ASC/DESC 排序字段] [Separator '分隔符'])
# https://blog.csdn.net/yz357823669/article/details/78794050

-- 14、查询没学过"张三"老师讲授的任一门课程的学生姓名
select sname from student
where sid not in(
	select sid from sc
	left join course on sc.cid = course.cid
	left join teacher on course.tid = teacher.tid
	where tname = '张三'
);

-- 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
select st.sid, sname, avg(score) from student as st
join sc using(sid)
where st.sid in(
	select sid from sc
	where sc.score < 60 or sc.score is null
	group by sc.sid
	having count(sc.sid) >= 2
)
group by st.sid;