Mysql經典四表查詢

**創建student表**

    屬性有:編號：id (主鍵，自動增加),姓名:sname,出生年月:sage,性別:ssex(枚舉)
    create table student(sid int primary key auto_increment,
    sname varchar(20),
    sage date,
    ssex enum(‘男’,‘女’));

**創建教師表**

     create table teacher(tid int primary key auto_increment,
     tname varchar(20));

**創建課程表**

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

**創建成績表**

    create table sc(sid int,
    cid int,
    score int);

**插入數據**

    1，先給student表插入數據
    insert into student values(1,'趙雷','1990-01-01','男'),
        (2,'錢電','1990-12-21','男'),
        (3,'孫風','1990-05-20','男'),
        (4,'李雲','1990-08-06','男'),
        (5,'周梅','1991-12-01','女'),
        (6,'吳蘭','1992-03-01','女'),
        (7,'鄭竹','1989-07-01','女'),
        (8,'王菊','1990-01-20','女');

    2, 給teacher表插入數據,這裏不能夠先給course表插入數據,由於course表外連接到teacher的主鍵
    insert into teacher values(1,'張三'),
            (2,'李四'),
            (3,'王五');

    3, 給course表插入數據
    insert into course values(1,'語文',2),
                (2,'語文',1),
                (3,'語文',3);

    4, 最後給sc表插入數據(題目圖片少了第1個學生成績，在這加上 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.查詢出只有兩門課程的所有學生的學號和姓名**

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

**2.查詢沒有學全全部課程的同窗的信息**

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

**3.查詢選修了所有課程的學生信息**

    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.查詢學過"張三"老師授課的同窗的信息**

    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='張三';

**5.查詢沒學過"張三"老師講授的任一門課程的學生姓名**

    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='張三');

**6.查詢平均成績大於等於85的全部學生的學號、姓名和平均成績**

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

**7.查詢任何一門課程成績在70分以上的姓名、課程名稱和分數**

    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.查詢不及格的課程**

    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.查詢課程編號爲01且課程成績在80分以上的學生的學號和姓名；**

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

**10.求每門課程的查詢選修了所有課程的學生信息學生人數**

      select cid,count(sid) from sc
      group by sc.cid;

**11.查詢學過編號爲"01"而且也學過編號爲"02"的課程的同窗的信息**

    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用於檢查子查詢是否至少會返回一行數據，該子查詢實際上並不返回任何數據，而是返回值True或False
    EXISTS 指定一個子查詢，檢測行的存在。

    //select 1 from table;與select anycol(目的表集合中的任意一行） from table;  表裏若是有記錄，就顯示 1  簡單理解就是不查詢具體列，只要有值就顯示1

**12.查詢"01"課程比"02"課程成績高的學生的信息及課程分數**

    SELECT s.*,a.score '課程01的分數',b.score '課程02的分數'
    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;圖片