MySQL经典50题

1、查询01课程比02课程成绩高的学生的信息及课程分数

#1.1查询01课程与02课程的课程表：
select student_id, score as c1_score from score where course_id='01';
select student_id, score as c2_score from score where course_id='02';

#1.2使用内连接，将两个课程表进行连接，并从中选出01
select c1.student_id,c1_score,c2_score from
	(select student_id, score as c1_score from score where course_id='01') as c1 inner join
	(select student_id, score as c2_score from score where course_id='02') as c2 
		on c1.student_id=c2.student_id
where c1_score>c2_score;

2、查询同时存在01课程和02课程的情况

select c1.student_id,c1_score,c2_score from
	(select student_id, score as c1_score from score where course_id='01') c1 inner join
	(select student_id, score as c2_score from score where course_id='02') c2 
		on c1.student_id=c2.student_id;

select c1.student_id, c1.score, c2.score 
	from score c1 join score c2 on c1.student_id=c2.student_id
where c1.course_id='01' and c2.course_id='02';

3、查询存在01课程但可能不存在02课程的情况(不存在时显示为 null )

select c1.student_id,c1_score,c2_score from
	(select student_id, score as c1_score from score where course_id='01') c1 left join
	(select student_id, score as c2_score from score where course_id='02') c2 
		on c1.student_id=c2.student_id;

4、查询不存在01课程但存在02课程的情况

select * from
	(select student_id, score as c1_score from score where course_id='01') c1 right join
	(select student_id, score as c2_score from score where course_id='02') c2 
		on c1.student_id=c2.student_id
where c1.student_id is null;

5、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.id,name,avg(score) from student left join score on student.id=student_id
	group by student_id
	having avg(score)>=60;

6、查询在score表存在成绩的学生信息

select distinct student.* from student right join score on id=student_id;

7、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(没成绩的显示为 null )

select id, name, count(course_id),sum(score) from student left join score on id=student_id
group by id;

8、查有成绩的学生信息

select distinct student.* from score left join student on id=student_id;

9、查询「李」姓老师的数量

select count(id) from teacher
where name like '李%';

10、查询学过「张三」老师授课的同学的信息

select student.* from student
left join score on student.id=student_id
left join course  on course_id=course.id
left join teacher on teacher_id=teacher.id
where teacher.name='张三';

11、查询没有学全所有课程的同学的信息

select student.* from student left join 
	(select student_id, count(course_id) as courseNum from score group by student_id)
	as studentCourse on student.id=studentCourse.student_id
where courseNum<(select count(id) as allNum 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 score on student.id=student_id
right join (select course_id from score where student_id='01') as No01Course on score.course_id=No01Course.course_id
where student.id!='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

#和01选了完全相同课程的同学，首先选出所选课程是01所选课程的子集的同学，再从中筛选出选课数量与01同学相同的。

#13.1 01同学选的所有课程：
select course_id from score where student_id='07';

#13.2 选了01没选的课的同学
select student_id from score 
where course_id not in
	(select course_id from score where student_id='07');
    
#13.3 排除掉这些人，就是01选课列表的子集的同学
select student_id from score
where student_id!='07' and student_id not in
	(select student_id from score 
		where course_id not in
		(select course_id from score where student_id='07'));

#13.4 选出（01选课列表的子集的同学）选课数量与01选课数量相同的人
select student_id from
	(select student_id from score
	where student_id!='07' and student_id not in
		(select student_id from score 
		where course_id not in
			(select course_id from score where student_id='07')
		)
	) as subset
group by student_id
having count(*)=
	(select count(course_id) from score where student_id='07');

14 查询没学过"张三"老师讲授的任一门课程的学生姓名

#14.1 查询学过张三教过的课的同学：
select student.id from student
left join score on student.id=student_id
left join course  on course_id=course.id
left join teacher on teacher_id=teacher.id
where teacher.name='张三';

#14.2 排除这些同学
select name from student
where id not in (
	select student.id from student
	left join score on student.id=student_id
	left join course  on course_id=course.id
	left join teacher on teacher_id=teacher.id
	where teacher.name='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

#15.1 选出不及格的课程和同学
select student_id from score
where score<60;

#15.2 从中选出count>=2的学生id
select student_id from
	(select student_id, score from score
	 where score<60
	) as sc 
group by student_id
having count(score)>=2;

#15.3 从中选出学号姓名和平均成绩
select student.id, student.name, avg(score) from
	student left join score on student.id=student_id
where student.id in (
	select student_id from
		(select student_id, score from score
		 where score<60
		) as sc 
	group by student_id
	having count(score)>=2
					)
group by student.id;

16 检索" 01 "课程分数小于 60，按分数降序排列的学生信息

select student.* from 
	student left join score on student.id=student_id
where course_id='01' and score<60
order by score desc;

17 按平均成绩从高到低显示所有学生的所有课程的成绩以及平均成绩

#选出所有同学各自的平均成绩
select student_id, avg(score) from score
group by student_id;

#连接至学生-成绩表
select student.id, student.name, score, avg_score  from 
	(student left join score on student.id=student_id)  left join
    (select student_id, avg(score) as avg_score from score 
		group by student_id
    ) as avgsc on student.id=avgsc.student_id
order by avg_score desc;