1、查询所有的课程的名称以及对应的任课老师姓名
select cname,tname from course left join teacher on course.teacher_id=teacher.tid;
2、查询学生表中男女生各有多少人
select gender,count(sid) from student group by gender;
3、查询物理成绩等于100的学生的姓名
#子查询的方式
select sname from student where sid in
(
select student_id from score
where course_id = (select cid from course where cname='物理') and num=100
);
#连表的方式
select sname from student inner join
(
select student_id from score
where course_id = (select cid from course where cname='物理') and num=100
) as a
on a.student_id=student.sid
;
4、查询平均成绩大于八十分的同学的姓名和平均成绩
select student.sname,t1.平均成绩 from student inner join
(select student_id,avg(num) 平均成绩 from score group by student_id having avg(num) > 80) as t1
on student.sid=t1.student_id;
5、查询所有学生的学号，姓名，选课数，总成绩
select student.sid,sname 学生名,选课数,总成绩 from student left join
(select student_id,count(course_id) 选课数,sum(num) 总成绩 from score group by student_id) as t1
on student.sid=t1.student_id
;
6、 查询姓李老师的个数
select count(tid) from teacher where tname like '李%';
7、 查询没有报李平老师课的学生姓名
select sname from student where sid not in (
select distinct student_id from score where course_id in (
select cid from course where teacher_id=(select tid from teacher where tname='李平老师')
)
);
8、 查询物理课程比生物课程高的学生的学号
select t1.student_id from
(select student_id,num from score inner join course on score.course_id=course.cid
where course.cname='物理') as t1
inner join
(select student_id,num from score inner join course on score.course_id=course.cid
where course.cname='生物') as t2
on t1.student_id=t2.student_id
where t1.num > t2.num
;
9、 查询没有同时选修物理课程和体育课程的学生姓名
select sname from student where sid in (
select student_id from score inner join cour
on course.cname in ('物理','体育') and course.cid=score.course_id
group by student_id having count(course_id) !=2
);
10、查询挂科超过两门(包括两门)的学生姓名和班级名字
select t2.sname,class.caption from
(select sname,class_id from student inner join (
select student_id from score
where num < 60 group by student_id having count(course_id) >=2
) as t1
on student.sid=t1.student_id) as t2
inner join class
on class.cid = t2.class_id
;
11 、查询选修了所有课程的学生姓名
select sname from student inner join
(
select student_id from score group by student_id having count(course_id) = (select count(cid) from course)
) t1
on t1.student_id = student.sid
;
12、查询李平老师教的课程的所有成绩记录
select student_id,course_id,num from score inner join
(
select cid from course inner join teacher on teacher.tname='李平老师' and teacher.tid=course.teacher_id
) as t1
on t1.cid=score.course_id
;
13、查询全部学生都选修了的课程号和课程名
select course.cid,course.cname from course inner join
(
select course_id from score group by course_id
having count(student_id) = (select count(sid) from student)
) t1
on t1.course_id=course.cid
;
14、查询每门课程被选修的次数
select course.cname,选修人数 from course inner join
(
select course_id,count(student_id) as 选修人数 from score group by course_id
) as t1
on t1.course_id=course.cid
;
15、查询之选修了一门课程的学生姓名和学号
select sid,sname from student inner join
(
select student_id from score group by student_id having count(course_id)=1
) t1
on t1.student_id = student.sid
;
16、查询所有学生考出的成绩并按从高到低排序（成绩去重）
select distinct num from score order by num desc;
17、查询平均成绩大于85的学生姓名和平均成绩
select student.sname,avg_num from student inner join
(
select student_id,avg(num) as avg_num from score group by student_id having avg(num) > 85
) t1
on student.sid=t1.student_id
;
18、查询生物成绩不及格的学生姓名和对应生物分数
select student.sname,t1.num from student inner join
(
select student_id,num from score
where course_id=(select cid from course where cname='生物') and num < 60
) t1
on t1.student_id=student.sid
;
19、查询在所有选修了李平老师课程的学生中，这些课程(李平老师的课程，不是所有课程)平均成绩最高的学生姓名
select sname from student where sid =
(
select student_id from score where course_id in
(
select cid from course inner join teacher on teacher.tname='李平老师' and course.teacher_id=teacher.tid
)
group by student_id
order by avg(num) desc
limit 1
);
