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| 1、查询所有的课程的名称以及对应的任课老师姓名
select course.cname,teacher.tname
from teacher
left join course on course.teacher_id=teacher.tid;
2、查询学生表中男女生各有多少人
select gender,count(gender)
from student
group by gender;
3、查询物理成绩等于100的学生的姓名
select student.sname
from course
join score on course.cid=score.course_id
join student on student.sid=score.student_id
where course.cname='物理' and score.num=100;
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,avg(score.num)
from student
join score on student.sid=score.student_id
group by student.sname
having avg(score.num)>80;
5、查询所有学生的学号，姓名，选课数，总成绩
select
student.sid,
student.sname,
count(course.cid),
sum(score.num)
from course
join score on course.cid=score.course_id
right join student on student.sid=score.student_id
group by student.sname
order by student.sid;
# ONLY_FULL_GROUP_BY 有限制 只选择出现在group by后面的列，或者给列增加聚合函数
# set sql_mode='STRICT_TRANS_TABLES,NO_ZERO_IN_DATE,NO_ZERO_DATE,ERROR_FOR_DIVISION_BY_ZERO,NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION';
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 student.sname from student where student.sname not in
(select
distinct(student.sname)
from course
join score on course.cid=score.course_id
right join student on student.sid=score.student_id
left join teacher on teacher.tid=course.teacher_id
where teacher.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 student
left join score on score.student_id=student.sid
left join course on course.cid=score.course_id
where course.cname='物理') as t1
join
(select
student_id,
num
from student
left join score on score.student_id=student.sid
left join course on course.cid=score.course_id
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
*
from student
left join score on score.student_id=student.sid
left join course on course.cid=score.course_id
where course.cid=2 or course.cid=3;
10、查询挂科超过两门(包括两门)的学生姓名和班级
select student.sname ,count(num)
from score
join student on student.sid=score.student_id
join class on class.cid=student.class_id
where score.num<60
group by student.sname
having count(num)>=2;
11、查询选修了所有课程的学生姓名
select
student.sname
from student
join score on score.student_id=student.sid
join course on course.cid=score.course_id
group by student.sname
having count(course.cname)=4;
12、查询李平老师教的课程的所有成绩记录
select
course.cname,
score.num,
student.sname
from course
join score on course.cid=score.course_id
right join student on student.sid=score.student_id
left join teacher on teacher.tid=course.teacher_id
where teacher.tname='李平老师';
13、查询全部学生都选修了的课程号和课程名
14、查询每门课程被选修的次数
15、查询之选修了一门课程的学生姓名和学号
16、查询所有学生考出的成绩并按从高到低排序（成绩去重）
17、查询平均成绩大于85的学生姓名和平均成绩
18、查询生物成绩不及格的学生姓名和对应生物分数
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
20、查询每门课程成绩最好的前两名学生姓名
21、查询不同课程但成绩相同的学号，课程号，成绩
22、查询没学过“叶平”老师课程的学生姓名以及选修的课程名称；
23、查询所有选修了学号为1的同学选修过的一门或者多门课程的同学学号和姓名；
24、任课最多的老师中学生单科成绩最高的学生姓名
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