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Join连接图
针对以上的多表查询，我们尝试做以下的实验：
Join实验：
CREATE TABLE `t_dept` (
`id` INT(11) NOT NULL AUTO_INCREMENT,
`deptName` VARCHAR(30) DEFAULT NULL,
`address` VARCHAR(40) DEFAULT NULL,
PRIMARY KEY (`id`)
) ENGINE=INNODB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8;
CREATE TABLE `t_emp` (
`id` INT(11) NOT NULL AUTO_INCREMENT,
`name` VARCHAR(20) DEFAULT NULL,
`age` INT(3) DEFAULT NULL,
`deptId` INT(11) DEFAULT NULL,
empno int not null,
PRIMARY KEY (`id`),
KEY `idx_dept_id` (`deptId`)
#CONSTRAINT `fk_dept_id` FOREIGN KEY (`deptId`) REFERENCES `t_dept` (`id`)
) ENGINE=INNODB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8;
INSERT INTO t_dept(deptName,address) VALUES('华山','华山');
INSERT INTO t_dept(deptName,address) VALUES('丐帮','洛阳');
INSERT INTO t_dept(deptName,address) VALUES('峨眉','峨眉山');
INSERT INTO t_dept(deptName,address) VALUES('武当','武当山');
INSERT INTO t_dept(deptName,address) VALUES('明教','光明顶');
INSERT INTO t_dept(deptName,address) VALUES('少林','少林寺');
INSERT INTO t_emp(NAME,age,deptId,empno) VALUES('风清扬',90,1,100001);
INSERT INTO t_emp(NAME,age,deptId,empno) VALUES('岳不群',50,1,100002);
INSERT INTO t_emp(NAME,age,deptId,empno) VALUES('令狐冲',24,1,100003);
INSERT INTO t_emp(NAME,age,deptId,empno) VALUES('洪七公',70,2,100004);
INSERT INTO t_emp(NAME,age,deptId,empno) VALUES('乔峰',35,2,100005);
INSERT INTO t_emp(NAME,age,deptId,empno) VALUES('灭绝师太',70,3,100006);
INSERT INTO t_emp(NAME,age,deptId,empno) VALUES('周芷若',20,3,100007);
INSERT INTO t_emp(NAME,age,deptId,empno) VALUES('张三丰',100,4,100008);
INSERT INTO t_emp(NAME,age,deptId,empno) VALUES('张无忌',25,5,100009);
INSERT INTO t_emp(NAME,age,deptId,empno) VALUES('韦小宝',18,null,100010);
ALTER TABLE `t_dept`
add CEO INT(11) ;
# CEO=2 值，都应该是t_emp 中id的值。
update t_dept set CEO=2 where id=1;
update t_dept set CEO=4 where id=2;
update t_dept set CEO=6 where id=3;
update t_dept set CEO=8 where id=4;
update t_dept set CEO=9 where id=5;
下面来看几个问题：
1 所有有门派的人员信息
mysql> select * from t_dept d left join t_emp e on d.id = e.deptId;
+----+----------+-----------+------+------+--------------+------+--------+--------+
| id | deptName | address | CEO | id | name | age | deptId | empno |
+----+----------+-----------+------+------+--------------+------+--------+--------+
| 1 | 华山 | 华山 | 2 | 1 | 风清扬 | 90 | 1 | 100001 |
| 1 | 华山 | 华山 | 2 | 2 | 岳不群 | 50 | 1 | 100002 |
| 1 | 华山 | 华山 | 2 | 3 | 令狐冲 | 24 | 1 | 100003 |
| 2 | 丐帮 | 洛阳 | 4 | 4 | 洪七公 | 70 | 2 | 100004 |
| 2 | 丐帮 | 洛阳 | 4 | 5 | 乔峰 | 35 | 2 | 100005 |
| 3 | 峨眉 | 峨眉山 | 6 | 6 | 灭绝师太 | 70 | 3 | 100006 |
| 3 | 峨眉 | 峨眉山 | 6 | 7 | 周芷若 | 20 | 3 | 100007 |
| 4 | 武当 | 武当山 | 8 | 8 | 张三丰 | 100 | 4 | 100008 |
| 5 | 明教 | 光明顶 | 9 | 9 | 张无忌 | 25 | 5 | 100009 |
| 6 | 少林 | 少林寺 | NULL | NULL | NULL | NULL | NULL | NULL |
+----+----------+-----------+------+------+--------------+------+--------+--------+
10 rows in set (0.04 sec)
2 列出所有用户，并显示其机构信息
mysql> select * from t_emp e left join t_dept d on d.id = e.deptId;
+----+--------------+------+--------+--------+------+----------+-----------+------+
| id | name | age | deptId | empno | id | deptName | address | CEO |
+----+--------------+------+--------+--------+------+----------+-----------+------+
| 1 | 风清扬 | 90 | 1 | 100001 | 1 | 华山 | 华山 | 2 |
| 2 | 岳不群 | 50 | 1 | 100002 | 1 | 华山 | 华山 | 2 |
| 3 | 令狐冲 | 24 | 1 | 100003 | 1 | 华山 | 华山 | 2 |
| 4 | 洪七公 | 70 | 2 | 100004 | 2 | 丐帮 | 洛阳 | 4 |
| 5 | 乔峰 | 35 | 2 | 100005 | 2 | 丐帮 | 洛阳 | 4 |
| 6 | 灭绝师太 | 70 | 3 | 100006 | 3 | 峨眉 | 峨眉山 | 6 |
| 7 | 周芷若 | 20 | 3 | 100007 | 3 | 峨眉 | 峨眉山 | 6 |
| 8 | 张三丰 | 100 | 4 | 100008 | 4 | 武当 | 武当山 | 8 |
| 9 | 张无忌 | 25 | 5 | 100009 | 5 | 明教 | 光明顶 | 9 |
| 10 | 韦小宝 | 18 | NULL | 100010 | NULL | NULL | NULL | NULL |
+----+--------------+------+--------+--------+------+----------+-----------+------+
10 rows in set (0.00 sec)
3 列出不入派的人员：
mysql> select * from t_emp where deptId is null;
+----+-----------+------+--------+--------+
| id | name | age | deptId | empno |
+----+-----------+------+--------+--------+
| 10 | 韦小宝 | 18 | NULL | 100010 |
+----+-----------+------+--------+--------+
1 row in set (0.01 sec)
mysql> select * from t_emp e left join t_dept d on e.deptId = d.id where e.deptId is null;
+----+-----------+------+--------+--------+------+----------+---------+------+
| id | name | age | deptId | empno | id | deptName | address | CEO |
+----+-----------+------+--------+--------+------+----------+---------+------+
| 10 | 韦小宝 | 18 | NULL | 100010 | NULL | NULL | NULL | NULL |
+----+-----------+------+--------+--------+------+----------+---------+------+
1 row in set (0.00 sec)
4 所有没人入的门派 ：
mysql> select * from t_dept d left join t_emp e on d.id = e.deptId where e.id is null;
+----+----------+-----------+------+------+------+------+--------+-------+
| id | deptName | address | CEO | id | name | age | deptId | empno |
+----+----------+-----------+------+------+------+------+--------+-------+
| 6 | 少林 | 少林寺 | NULL | NULL | NULL | NULL | NULL | NULL |
+----+----------+-----------+------+------+------+------+--------+-------+
1 row in set (0.00 sec)
5 列出所有人员和门派的对照关系
mysql> select * from t_emp e left join t_dept d on e.deptId = d.id
-> union
-> select * from t_emp e right join t_dept d on e.deptId = d.id;
+------+--------------+------+--------+--------+------+----------+-----------+------+
| id | name | age | deptId | empno | id | deptName | address | CEO |
+------+--------------+------+--------+--------+------+----------+-----------+------+
| 1 | 风清扬 | 90 | 1 | 100001 | 1 | 华山 | 华山 | 2 |
| 2 | 岳不群 | 50 | 1 | 100002 | 1 | 华山 | 华山 | 2 |
| 3 | 令狐冲 | 24 | 1 | 100003 | 1 | 华山 | 华山 | 2 |
| 4 | 洪七公 | 70 | 2 | 100004 | 2 | 丐帮 | 洛阳 | 4 |
| 5 | 乔峰 | 35 | 2 | 100005 | 2 | 丐帮 | 洛阳 | 4 |
| 6 | 灭绝师太 | 70 | 3 | 100006 | 3 | 峨眉 | 峨眉山 | 6 |
| 7 | 周芷若 | 20 | 3 | 100007 | 3 | 峨眉 | 峨眉山 | 6 |
| 8 | 张三丰 | 100 | 4 | 100008 | 4 | 武当 | 武当山 | 8 |
| 9 | 张无忌 | 25 | 5 | 100009 | 5 | 明教 | 光明顶 | 9 |
| 10 | 韦小宝 | 18 | NULL | 100010 | NULL | NULL | NULL | NULL |
| NULL | NULL | NULL | NULL | NULL | 6 | 少林 | 少林寺 | NULL |
+------+--------------+------+--------+--------+------+----------+-----------+------+
11 rows in set (0.00 sec)
注意：union可以去掉重复的数据
6 列出所有没入派的人员和没人入的门派
mysql> select * from t_emp e left join t_dept d on e.deptId = d.id where e.deptId is null
-> union
-> select * from t_emp e right join t_dept d on e.deptId = d.id where e.id is null;
+------+-----------+------+--------+--------+------+----------+-----------+------+
| id | name | age | deptId | empno | id | deptName | address | CEO |
+------+-----------+------+--------+--------+------+----------+-----------+------+
| 10 | 韦小宝 | 18 | NULL | 100010 | NULL | NULL | NULL | NULL |
| NULL | NULL | NULL | NULL | NULL | 6 | 少林 | 少林寺 | NULL |
+------+-----------+------+--------+--------+------+----------+-----------+------+
2 rows in set (0.00 sec)
7，求各个门派对应的掌门人名称:
mysql> select * from t_dept d left join t_emp e on d.ceo = e.id;
+----+----------+-----------+------+------+--------------+------+--------+--------+
| id | deptName | address | CEO | id | name | age | deptId | empno |
+----+----------+-----------+------+------+--------------+------+--------+--------+
| 1 | 华山 | 华山 | 2 | 2 | 岳不群 | 50 | 1 | 100002 |
| 2 | 丐帮 | 洛阳 | 4 | 4 | 洪七公 | 70 | 2 | 100004 |
| 3 | 峨眉 | 峨眉山 | 6 | 6 | 灭绝师太 | 70 | 3 | 100006 |
| 4 | 武当 | 武当山 | 8 | 8 | 张三丰 | 100 | 4 | 100008 |
| 5 | 明教 | 光明顶 | 9 | 9 | 张无忌 | 25 | 5 | 100009 |
| 6 | 少林 | 少林寺 | NULL | NULL | NULL | NULL | NULL | NULL |
+----+----------+-----------+------+------+--------------+------+--------+--------+
6 rows in set (0.00 sec)
mysql> select * from t_dept d inner join t_emp e on d.ceo = e.id;
+----+----------+-----------+------+----+--------------+------+--------+--------+
| id | deptName | address | CEO | id | name | age | deptId | empno |
+----+----------+-----------+------+----+--------------+------+--------+--------+
| 1 | 华山 | 华山 | 2 | 2 | 岳不群 | 50 | 1 | 100002 |
| 2 | 丐帮 | 洛阳 | 4 | 4 | 洪七公 | 70 | 2 | 100004 |
| 3 | 峨眉 | 峨眉山 | 6 | 6 | 灭绝师太 | 70 | 3 | 100006 |
| 4 | 武当 | 武当山 | 8 | 8 | 张三丰 | 100 | 4 | 100008 |
| 5 | 明教 | 光明顶 | 9 | 9 | 张无忌 | 25 | 5 | 100009 |
+----+----------+-----------+------+----+--------------+------+--------+--------+
5 rows in set (0.01 sec)
8，求所有当上掌门人的平均年龄:
mysql> select avg(age) from t_dept d inner join t_emp e on d.ceo = e.id;
+----------+
| avg(age) |
+----------+
| 63.0000 |
+----------+
1 row in set (0.01 sec)
mysql> select avg(age) from t_dept d inner join t_emp e on d.ceo = e.id;
+----------+
| avg(age) |
+----------+
| 63.0000 |
+----------+
1 row in set (0.00 sec)
聚合函数能够自动去重。
9，求所有人物对应的掌门名称:
mysql> select e1.id,e1.name,d.deptName,d.address,e2.name as DeptName from t_emp e1 left join t_dept d on e1.deptId = d.id left join t_emp e2 on d.ceo = e2.id;
+----+--------------+----------+-----------+--------------+
| id | name | deptName | address | DeptName |
+----+--------------+----------+-----------+--------------+
| 1 | 风清扬 | 华山 | 华山 | 岳不群 |
| 2 | 岳不群 | 华山 | 华山 | 岳不群 |
| 3 | 令狐冲 | 华山 | 华山 | 岳不群 |
| 4 | 洪七公 | 丐帮 | 洛阳 | 洪七公 |
| 5 | 乔峰 | 丐帮 | 洛阳 | 洪七公 |
| 6 | 灭绝师太 | 峨眉 | 峨眉山 | 灭绝师太 |
| 7 | 周芷若 | 峨眉 | 峨眉山 | 灭绝师太 |
| 8 | 张三丰 | 武当 | 武当山 | 张三丰 |
| 9 | 张无忌 | 明教 | 光明顶 | 张无忌 |
| 10 | 韦小宝 | NULL | NULL | NULL |
+----+--------------+----------+-----------+--------------+
10 rows in set (0.00 sec)