多表查询（使用连接和外键） 示例

create table class(cid int primary key auto_increment,
                  caption char(4) not null)charset utf8;
                  
create table teacher(tid int primary key auto_increment,
                    tname varchar(32) not null)charset utf8;
                    
create table student(sid int primary key auto_increment,
                    sname varchar(32) not null,
                    gender enum('male','female','others') not null default 'male',
                    class_id int,
                    foreign key(class_id) references class(cid)
                    on update cascade
                    on delete cascade) charset utf8;
 
create table course(cid int primary key auto_increment,
                   cname varchar(16) not null,
                   teacher_id int,
                   foreign key(teacher_id) references teacher(tid)
                   on update cascade
                   on delete cascade) charset utf8;
                   
create table score(sid int primary key auto_increment,
                  student_id int,
                  course_id int,
                  number int(3) not null,
                  foreign key(student_id) references student(sid)
                   on update cascade
                   on delete cascade,
                   foreign key(course_id) references course(cid)
                   on update cascade
                   on delete cascade) charset utf8;
                   
insert into class(caption) values('三年二班'),('一年三班'),('三年一班');
insert into teacher(tname) values('波多'),('苍空'),('饭岛');
insert into student(sname,gender,class_id) values('钢蛋','female',1),('铁锤','female',1),('山炮','male',2);
insert into course(cname,teacher_id) values('生物',1),('体育',1),('物理',2);
insert into score(student_id,course_id,number) values(1,1,60),(1,2,59),(2,2,100);


1. 查询所有大于60分的学生的姓名和学号  (DISTINCT: 去重)
select  distinct student.sid,sname from student where student.sid in (select student_id from score where number>60);

2.查询每个老师教授的课程数量 和 老师信息#################
select tid,count(cname) from teacher left join course on teacher.tid=course.teacher_id group by teacher.tid;

3. 查询学生的信息以及学生所在的班级信息
select sid,sname,gender,caption from student left join class on student.class_id=class.cid;

4、学生中男生的个数和女生的个数
select gender,count(gender) from student group by gender;

5、获取所有学习'生物'的学生的学号和成绩；姓名
select student.sid,student.sname,score.number from student left join score on student.sid=score.student_id where score.course_id=(select cid from course where cname='生物');
  
6、查询平均成绩大于60分的同学的学号和平均成绩； 
select student_id,avg(number) from score group by student_id having avg(number)>60; 

7、查询姓“李”的老师的个数；
select count(tname) from teacher where tname like '李%';
8、查询课程成绩小于60分的同学的学号、姓名；
select student.sid,student.sname from student where student.sid in(select student_id from score where number<60);
9. 删除学习“叶平”老师课的SC表记录
delete from score where course_id in (select course.cid from course where teacher_id=(select teacher.tid from teacher where tname='叶平'));

10.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.cid,max(number),min(number) from course left join score on course.cid=score.course_id group by course.cid;
11.查询每门课程被选修的学生数
select course_id,count(student_id) from score group by course_id;
12.查询姓“张”的学生名单；
select sname from student where sname like '张%';

13.查询每门课程的平均成绩，结果按平均成绩升序排列，平均成绩相同时，按课程号降序排列
select course_id,avg(number) from score where course_id in (select course.cid from course) group by course_id order by avg(number) asc,course_id desc;
14.查询平均成绩大于85的所有学生的学号、姓名和平均成绩 ####################
select student.sid,student.sname,avg(number) from student left join score on student.sid=score.student_id group by student.sid having avg(number)>85;
15.查询课程编号为3且课程成绩在80分以上的学生的学号和姓名；
select student_id,student.sname from student left join score on score.student_id=student.sid where score.course_id=3 and number>80; 
16.查询各个课程及相应的选修人数
select course.cname,count(student_id) from course left join score on course.cid=score.course_id group by course.cname;
17.查询“4”课程分数小于60，按分数降序排列的同学学号
select student_id from score where course_id=4 and number<60 order by number desc;
18.删除学号为“2”的同学的“1”课程的成绩
delete from score where student_id=2 and course_id=1;