多表连接查询
-- 1. 预备工作
CREATE DATABASE school CHARSET utf8;
USE school
CREATE TABLE student(
sno INT NOT NULL PRIMARY KEY AUTO_INCREMENT COMMENT '学号',
sname VARCHAR(20) NOT NULL COMMENT '姓名',
sage TINYINT UNSIGNED NOT NULL COMMENT '年龄',
ssex ENUM('f','m') NOT NULL DEFAULT 'm' COMMENT '性别'
)ENGINE=INNODB CHARSET=utf8;
CREATE TABLE course(
cno INT NOT NULL PRIMARY KEY COMMENT '课程编号',
cname VARCHAR(20) NOT NULL COMMENT '课程名字',
tno INT NOT NULL COMMENT '教师编号'
)ENGINE=INNODB CHARSET utf8;
CREATE TABLE sc (
sno INT NOT NULL COMMENT '学号',
cno INT NOT NULL COMMENT '课程编号',
score INT NOT NULL DEFAULT 0 COMMENT '成绩'
)ENGINE=INNODB CHARSET=utf8;
CREATE TABLE teacher(
tno INT NOT NULL PRIMARY KEY COMMENT '教师编号',
tname VARCHAR(20) NOT NULL COMMENT '教师名字'
)ENGINE=INNODB CHARSET utf8;
INSERT INTO student(sno,sname,sage,ssex)
VALUES
(1,'zhang3',18,'m'),
(2,'zhang4',18,'m'),
(3,'li4',18,'m'),
(4,'wang5',19,'f'),
(5,'zh4',18,'m'),
(6,'zhao4',18,'m'),
(7,'ma6',19,'f'),
(8,'oldboy',20,'m'),
(9,'oldgirl',20,'f'),
(10,'oldp',25,'m');
INSERT INTO teacher(tno,tname) VALUES
(101,'oldboy'),
(102,'hesw'),
(103,'oldguo')
(104,'alex');
INSERT INTO course(cno,cname,tno)
VALUES
(1001,'linux',101),
(1002,'python',102),
(1003,'mysql',103)
(1004,'go',105);
INSERT INTO sc(sno,cno,score)
VALUES
(1,1001,80),
(1,1002,59),
(2,1002,90),
(2,1003,100),
(3,1001,99),
(3,1003,40),
(4,1001,79),
(4,1002,61),
(4,1003,99),
(5,1003,40),
(6,1001,89),
(6,1003,77),
(7,1001,67),
(7,1003,82),
(8,1001,70),
(9,1003,80),
(10,1003,96);
-- 2. 多表连接的类型
-- 笛卡尔乘积
mysql> select * from teacher , course;
+-----+--------+------+--------+-----+
| tno | tname | cno | cname | tno |
+-----+--------+------+--------+-----+
| 101 | oldboy | 1001 | linux | 101 |
| 102 | hesw | 1001 | linux | 101 |
| 103 | oldguo | 1001 | linux | 101 |
| 104 | alex | 1001 | linux | 101 |
| 101 | oldboy | 1002 | python | 102 |
| 102 | hesw | 1002 | python | 102 |
| 103 | oldguo | 1002 | python | 102 |
| 104 | alex | 1002 | python | 102 |
| 101 | oldboy | 1003 | mysql | 103 |
| 102 | hesw | 1003 | mysql | 103 |
| 103 | oldguo | 1003 | mysql | 103 |
| 104 | alex | 1003 | mysql | 103 |
| 101 | oldboy | 1004 | go | 105 |
| 102 | hesw | 1004 | go | 105 |
| 103 | oldguo | 1004 | go | 105 |
| 104 | alex | 1004 | go | 105 |
+-----+--------+------+--------+-----+
-- 内连接 （取交集）
SQL92:
mysql> select *from teacher , course where teacher.tno=course.tno;
SQL99:
mysql> select *from teacher join course on teacher.tno=course.tno;
+-----+--------+------+--------+-----+
| tno | tname | cno | cname | tno |
+-----+--------+------+--------+-----+
| 101 | oldboy | 1001 | linux | 101 |
| 102 | hesw | 1002 | python | 102 |
| 103 | oldguo | 1003 | mysql | 103 |
+-----+--------+------+--------+-----+
-- 外连接 （左、右）
mysql>
mysql> select * from teacher left join course on teacher.tno = course.tno;
+-----+--------+------+--------+------+
| tno | tname | cno | cname | tno |
+-----+--------+------+--------+------+
| 101 | oldboy | 1001 | linux | 101 |
| 102 | hesw | 1002 | python | 102 |
| 103 | oldguo | 1003 | mysql | 103 |
| 104 | alex | NULL | NULL | NULL |
+-----+--------+------+--------+------+
4 rows in set (0.00 sec)
mysql> select * from teacher right join course on teacher.tno = course.tno;
+------+--------+------+--------+-----+
| tno | tname | cno | cname | tno |
+------+--------+------+--------+-----+
| 101 | oldboy | 1001 | linux | 101 |
| 102 | hesw | 1002 | python | 102 |
| 103 | oldguo | 1003 | mysql | 103 |
| NULL | NULL | 1004 | go | 105 |
+------+--------+------+--------+-----+
-- 3. 多表连接语法格式
select xxx
from a join b
on a.x=b.y
where
group by
having
order by
limit
select xxx
from a join b
on a.x=b.y
join c
on b.z=c.zz
核心思路：
1.找到所有相关表
2.找到所有表之间的关联关系
3.罗列其他的查询条件
--3. 多表连接例子：
--- 统计学员zhang3,学习了几门课
SELECT student.sname,COUNT(*)
FROM student JOIN sc
ON student.sno =sc.sno
WHERE student.sname='zhang3';
--- 所有学员学习的课程门数
SELECT student.sname,COUNT(*)
FROM student JOIN sc
ON student.sno =sc.sno
GROUP BY student.sname;
--- 查询zhang3,学习的课程名称有哪些?
SELECT student.sname,GROUP_CONCAT(course.cname)
FROM student
JOIN sc
ON student.sno=sc.sno
JOIN course
ON sc.cno=course.cno
WHERE student.sname = 'zhang3'
GROUP BY student.sname;
--- 查询oldguo所教课程的平均分数
SELECT teacher.tname ,AVG(sc.score) FROM
teacher
JOIN course
ON teacher.tno=course.tno
JOIN sc
ON course.cno=sc.cno
WHERE teacher.tname='oldguo'
GROUP BY teacher.tname;
--- 每位老师所教课程的平均分,并按平均分排序
SELECT teacher.tname ,AVG(sc.score) FROM
teacher
JOIN course
ON teacher.tno=course.tno
JOIN sc
ON course.cno=sc.cno
GROUP BY teacher.tname
ORDER BY AVG(sc.score);
--- 查询oldguo所教的不及格的学生姓名
SELECT teacher.tname,GROUP_CONCAT(student.sname) FROM
teacher
JOIN course
ON teacher.tno=course.tno
JOIN sc
ON course.cno=sc.cno
JOIN student
ON sc.sno=student.sno
WHERE teacher.tname='oldguo' AND sc.score<60
GROUP BY teacher.tname ;
--- 查询所有老师所教学生不及格的信息
SELECT teacher.tname,GROUP_CONCAT(student.sname) FROM
teacher
JOIN course
ON teacher.tno=course.tno
JOIN sc
ON course.cno=sc.cno
JOIN student
ON sc.sno=student.sno
WHERE sc.score<60
GROUP BY teacher.tname ;
别名的使用
-- 表别名
SELECT a.tname,GROUP_CONCAT(d.sname) FROM
teacher AS a
JOIN course AS b
ON a.tno=b.tno
JOIN sc AS c
ON b.cno=c.cno
JOIN student AS d
ON c.sno=d.sno
WHERE c.score<60
GROUP BY a.tname ;
-- 列别名
SELECT a.tname AS aa ,GROUP_CONCAT(d.sname) AS bb FROM
teacher AS a
JOIN course AS b
ON a.tno=b.tno
JOIN sc AS c
ON b.cno=c.cno
JOIN student AS d
ON c.sno=d.sno
WHERE c.score<60
GROUP BY a.tname ;
SELECT teacher.tname AS '老师名' ,AVG(sc.score) AS '平均分' FROM
teacher
JOIN course
ON teacher.tno=course.tno
JOIN sc
ON course.cno=sc.cno
GROUP BY teacher.tname
ORDER BY 平均分;