mysql多表查询前15题https://www.cnblogs.com/Eva-J/articles/9688383.html

# 1、查询男生、女生的人数；
#    select gender,count(*) from student group by gender;
# 2、查询姓“张”的学生名单；
#    select * from student where sname like '张%';
# 3、课程平均分从高到低显示
#    select course_id,avg(num) as a from score group by course_id order by a desc; #查出了各个部门的平均分
# 4、查询有课程成绩小于60分的同学的学号、姓名；
#    select sid,sname from student where sid in (select distinct student_id from score where num<60);
# 5、查询至少有一门课与学号为1的同学所学课程相同的同学的学号和姓名；
#    学号1学的课程  select course_id from score where student_id =1;
#    相同课程的学号 select distinct student_id from score where course_id in (select course_id from score where student_id =1);
#              select sid,sname from student where sid in(select distinct student_id from score where course_id in (select course_id from score where student_id =1) and student_id !=1);
# 6、查询出只选修了一门课程的全部学生的学号和姓名；
#   #学生们的选课情况   select student_id from score group by student_id having count(sid)=1;
#   # having          select sname,sid from student where sid =(select student_id from score group by student_id having count(sid)=1);
# 7、查询各科成绩最高和最低的分：以如下形式显示：课程ID，最高分，最低分；
#         select course_id,max(num),min(num) from score group by course_id;
# 8、查询课程编号“2”的成绩比课程编号“1”课程低的所有同学的学号、姓名；
#         把课程1,2的所有人的学生id,成绩找出来
#         select student_id as sid1,num as n1 from score where course_id =1 ;
#         select student_id as sid2,num as n2 from score where course_id =2 ;
#         取课程2成绩比课程1低的所有同学的学好
#          select sid1 from(  select student_id as sid1,num as n1 from score where course_id =1)as t1
#          inner join
#          (select student_id as sid2,num as n2 from score where course_id =2) as t2
#          on t1.sid1=t2.sid2 where n2<n1;
#         连表student表取名字和学号
#         select sid,sname from student right join(
#         select sid1 from(  select student_id as sid1,num as n1 from score where course_id =1)as t1
#         inner join
#        (select student_id as sid2,num as n2 from score where course_id =2) as t2
#        on t1.sid1=t2.sid2 where n2<n1
#        )as tmp
#        on student.sid = tmp.sid1;
# 9、查询“生物”课程比“物理”课程成绩高的所有学生的学号；
#       现获取生生物课,物理课的id
#       select cid from course where cname='生物';
#       select cid from course where cname='物理';
#       找到所有血生物,物理的人的学号和分数
#       select student_id as sid1,num as n1 from score where course_id=(select cid from course where cname='生物')
#       select student_id as sid2,num as n2 from score where course_id=(select cid from course where cname='物理')
#       select sid from student right join(
#       select sid1 from (select student_id as sid1,num as n1 from score where course_id=(select cid from course where cname='生物')) as t1
#       inner join
#       (select student_id as sid2,num as n2 from score where course_id=(select cid from course where cname='物理'))as t2
#       on t1.sid1=t2.sid2 where n1 >n2
#       )as tmp
#       on student.sid=tmp.sid1
# 10、查询平均成绩大于60分的同学的学号和平均成绩;
#      select sid,avg(num) from score group by student_id having avg(num)>60;
# 11、查询所有同学的学号、姓名、选课数、总成绩；
#      select student_id,count(course_id)as '选课数',sum(num)as '总成绩' from score group by student_id
#      select student_id,sname,c,b from student as t1
#      inner join
#      (select student_id,count(course_id)as c,sum(num)as b from score group by student_id)as t2
#       on t1.sid=t2.student_id
# 12、查询姓“李”的老师的个数；
#     select count(tid) from teacher  where tname like '李%';
# 13、查询没学过“张磊老师”课的同学的学号、姓名；
#     张磊老师教的课程 select cid from course where teacher_id=(select tid from teacher where tname ='张磊老师');
#     学过张磊老师课的同学的student_id    select student_id from score where course_id=( select cid from course where teacher_id=(select tid from teacher where tname ='张磊老师'));
#     没学过的同学id          select sid,sname from student where sid not in (select student_id from score where course_id=( select cid from course where teacher_id=(select tid from teacher where tname ='张磊老师')));
# 14、查询学过“1”并且也学过编号“2”课程的同学的学号、姓名；
#     学过1的同学 select student_id from score where course_id =1;
#     是否学过2的同学学号   select student_id from score where student_id in (select student_id from score where course_id =1) and course_id=2;
#     姓名      select sid,sname from student where sid in(select student_id from score where student_id in (select student_id from score where course_id =1) and course_id=2);
# 15、查询学过“李平老师”所教的所有课的同学的学号、姓名；
#     李平老师教的课  select cid from course where teacher_id=(select tid from teacher where tname='李平老师');
#     学过李平老师课的学生 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'));
#     学生的学号和姓名    select sid,sname from student where sid 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')));

原文地址：https://www.cnblogs.com/zhaolei1986/p/12640660.html