Mysql经典四表查询

**建立student表**

    属性有:编号：id (主键，自动增长),姓名:sname,出生年月:sage,性别:ssex(枚举)
    create table student(sid int primary key auto_increment,
    sname varchar(20),
    sage date,
    ssex enum(‘男’,‘女’));

**建立教师表**
    
     create table teacher(tid int primary key auto_increment,
     tname varchar(20));

**建立课程表**

    create table course(cid int primary key auto_increment,
    cname varchar(20),
    tid int,
    foreign key(tid) references teacher(tid));

**建立成绩表**
    
    create table sc(sid int,
    cid int,
    score int);

 **插入数据**   

    1，先给student表插入数据
    insert into student values(1,'赵雷','1990-01-01','男'),
        (2,'钱电','1990-12-21','男'),
        (3,'孙风','1990-05-20','男'),
        (4,'李云','1990-08-06','男'),
        (5,'周梅','1991-12-01','女'),
        (6,'吴兰','1992-03-01','女'),
        (7,'郑竹','1989-07-01','女'),
        (8,'王菊','1990-01-20','女');
    
    2, 给teacher表插入数据,这里不可以先给course表插入数据,因为course表外链接到teacher的主键
    insert into teacher values(1,'张三'),
            (2,'李四'),
            (3,'王五');
    
    3, 给course表插入数据
    insert into course values(1,'语文',2),
                (2,'语文',1),
                (3,'语文',3);
    
    4, 最后给sc表插入数据(题目图片少了第1个学生成绩，在这加上 1,1,90;  1,2,80;  1,3,90)
    insert into sc values(1,1,90),
                (1,2,80),
                (1,3,90),
                (2,1,70),
                (2,2,60),
                (2,3,80),
                (3,1,80),
                (3,2,80),
                (3,3,80),
                (4,1,50),
                (4,2,30),
                (4,3,20),
                (5,1,76),
                (5,2,87),
                (6,1,31),
                (6,3,34),
                (7,2,89),
                (7,3,98);

**1.查询出只有两门课程的全部学生的学号和姓名**

    SELECT * FROM student s
    WHERE s.sid in
    (SELECT sc.sid FROM sc
     GROUP BY sc.sid
    HAVING COUNT(*)=2);

**2.查询没有学全所有课程的同学的信息**
    
    SELECT s.sname,cname,score FROM student s,course,sc
    WHERE sc.sid=s.sid
    AND sc.cid=course.cid;
    
**3.查询选修了全部课程的学生信息**

    SELECT * FROM student s
    WHERE s.sid in
    (SELECT sc.sid from sc
    GROUP BY sc.sid
    HAVING COUNT(*)=(SELECT COUNT(*) from course));
    
    ------------------------------------
    SELECT * FROM student
    WHERE NOT EXISTS
    (SELECT * FROM course
      WHERE NOT EXISTS 
     (SELECT * 
      FROM sc
    WHERE sc.sid=student.sid AND sc.cid=course.cid));

**4.查询学过"张三"老师授课的同学的信息**
    
    SELECT * FROM student  s,sc,teacher t, course
    WHERE sc.sid=s.sid
    AND course.cid=sc.cid
    AND course.tid=t.tid
    AND  t.tname='张三';

**5.查询没学过"张三"老师讲授的任一门课程的学生姓名**
    
    SELECT * FROM student s
    WHERE s.sid not in
    (SELECT sc.sid FROM sc ,course,teacher 
    WHERE  sc.cid=course.cid 
    AND teacher.tid=course.tid AND teacher.tname='张三');

**6.查询平均成绩大于等于85的所有学生的学号、姓名和平均成绩**

    SELECT s.sid,s.sname,AVG(score) from sc ,student s
    WHERE  s.sid=sc.sid 
    GROUP BY sid 
    HAVING avg(score)>80;

**7.查询任何一门课程成绩在70分以上的姓名、课程名称和分数**

    SELECT s.sid,s.sname,c.cname,score 
    FROM student s,sc,course c
    WHERE sc.sid=s.sid 
    AND sc.cid=c.cid
    AND score>70;
**8.查询不及格的课程**

    SELECT s.sname,c.cname,score FROM student s,course c,sc
    WHERE sc.sid=s.sid
    AND sc.cid=c.cid
    AND sc.score<60;

**9.查询课程编号为01且课程成绩在80分以上的学生的学号和姓名；**

    SELECT s.sid,s.sname FROM student s ,sc
    WHERE s.sid=sc.sid
    AND cid=1
    AND score>80;

**10.求每门课程的查询选修了全部课程的学生信息学生人数**
      
      select cid,count(sid) from sc 
      group by sc.cid;

**11.查询学过编号为"01"并且也学过编号为"02"的课程的同学的信息**

    SELECT s.* FROM student s,sc 
    WHERE sc.sid=s.sid
    AND sc.cid=01
    AND EXISTS
    (SELECT 1 FROM sc sc1
    WHERE  sc1.sid=sc.sid 
    AND sc1.cid=02);
    
    
    //EXISTS用于检查子查询是否至少会返回一行数据，该子查询实际上并不返回任何数据，而是返回值True或False
    EXISTS 指定一个子查询，检测 行 的存在。

    //select 1 from table;与select anycol(目的表集合中的任意一行） from table;  表里如果有记录，就显示 1  简单理解就是不查询具体列，只要有值就显示1

**12.查询"01"课程比"02"课程成绩高的学生的信息及课程分数**

    SELECT s.*,a.score '课程01的分数',b.score '课程02的分数'
    FROM student s,sc a,sc b
    WHERE   a.sid=s.sid
    AND b.sid=s.sid
    AND a.cid=01
    AND b.cid=02
    AND a.score>b.score;