mysql四-2：多表查询

Posted 2020-11-04 休耕

一、介绍

本节主题：

• 多表连接查询
• 复合条件连接查询
• 子查询

准备表：

#建表
create table department(
id int,
name varchar(20) 
);

create table employee(
id int primary key auto_increment,
name varchar(20),
sex enum(‘male‘,‘female‘) not null default ‘male‘,
age int,
dep_id int
);

#插入数据
insert into department values
(200,‘技术‘),
(201,‘人力资源‘),
(202,‘销售‘),
(203,‘运营‘);

insert into employee(name,sex,age,dep_id) values
(‘egon‘,‘male‘,18,200),
(‘alex‘,‘female‘,48,201),
(‘wupeiqi‘,‘male‘,38,201),
(‘yuanhao‘,‘female‘,28,202),
(‘liwenzhou‘,‘male‘,18,200),
(‘jingliyang‘,‘female‘,18,204)
;


#查看表结构和数据
mysql> desc department;
+-------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id | int(11) | YES | | NULL | |
| name | varchar(20) | YES | | NULL | |
+-------+-------------+------+-----+---------+-------+

mysql> desc employee;
+--------+-----------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+--------+-----------------------+------+-----+---------+----------------+
| id | int(11) | NO | PRI | NULL | auto_increment |
| name | varchar(20) | YES | | NULL | |
| sex | enum(‘male‘,‘female‘) | NO | | male | |
| age | int(11) | YES | | NULL | |
| dep_id | int(11) | YES | | NULL | |
+--------+-----------------------+------+-----+---------+----------------+

mysql> select * from department;
+------+--------------+
| id | name |
+------+--------------+
| 200 | 技术 |
| 201 | 人力资源 |
| 202 | 销售 |
| 203 | 运营 |
+------+--------------+

mysql> select * from employee;
+----+------------+--------+------+--------+
| id | name | sex | age | dep_id |
+----+------------+--------+------+--------+
| 1 | egon | male | 18 | 200 |
| 2 | alex | female | 48 | 201 |
| 3 | wupeiqi | male | 38 | 201 |
| 4 | yuanhao | female | 28 | 202 |
| 5 | liwenzhou | male | 18 | 200 |
| 6 | jingliyang | female | 18 | 204 |
+----+------------+--------+------+--------+

创建department和employee表，并写入数据

　　需要注意的是在employee表有一个dep_id=204是department表没有的。同时，department表有一个id=203在employee表没有对应的记录。

二、多表连接查询

#重点：外链接语法

SELECT 字段列表
    FROM 表1 INNER|LEFT|RIGHT JOIN 表2
    ON 表1.字段 = 表2.字段;

1、交叉连接：不适用任何匹配条件。生成笛卡尔积

mysql> select * from employee, department;
+----+------------+--------+------+--------+------+--------------+
| id | name       | sex    | age  | dep_id | id   | name         |
+----+------------+--------+------+--------+------+--------------+
|  1 | egon       | male   |   18 |    200 |  200 | 技术         |
|  1 | egon       | male   |   18 |    200 |  201 | 人力资源     |
|  1 | egon       | male   |   18 |    200 |  202 | 销售         |
|  1 | egon       | male   |   18 |    200 |  203 | 运营         |
|  2 | alex       | female |   48 |    201 |  200 | 技术         |
|  2 | alex       | female |   48 |    201 |  201 | 人力资源     |
|  2 | alex       | female |   48 |    201 |  202 | 销售         |
|  2 | alex       | female |   48 |    201 |  203 | 运营         |
|  3 | wupeiqi    | male   |   38 |    201 |  200 | 技术         |
|  3 | wupeiqi    | male   |   38 |    201 |  201 | 人力资源     |
|  3 | wupeiqi    | male   |   38 |    201 |  202 | 销售         |
|  3 | wupeiqi    | male   |   38 |    201 |  203 | 运营         |
|  4 | yuanhao    | female |   28 |    202 |  200 | 技术         |
|  4 | yuanhao    | female |   28 |    202 |  201 | 人力资源     |
|  4 | yuanhao    | female |   28 |    202 |  202 | 销售         |
|  4 | yuanhao    | female |   28 |    202 |  203 | 运营         |
|  5 | liwenzhou  | male   |   18 |    200 |  200 | 技术         |
|  5 | liwenzhou  | male   |   18 |    200 |  201 | 人力资源     |
|  5 | liwenzhou  | male   |   18 |    200 |  202 | 销售         |
|  5 | liwenzhou  | male   |   18 |    200 |  203 | 运营         |
|  6 | jingliyang | female |   18 |    204 |  200 | 技术         |
|  6 | jingliyang | female |   18 |    204 |  201 | 人力资源     |
|  6 | jingliyang | female |   18 |    204 |  202 | 销售         |
|  6 | jingliyang | female |   18 |    204 |  203 | 运营         |
+----+------------+--------+------+--------+------+--------------+
24 rows in set (0.01 sec)

　　每个员工和四个部门做一次对应关系。但这四次匹配仅有一次是需要的：即dep_id和id相同的那个。

mysql> select * from employee, department where employee.dep_id = department.id;
+----+-----------+--------+------+--------+------+--------------+
| id | name      | sex    | age  | dep_id | id   | name         |
+----+-----------+--------+------+--------+------+--------------+
|  1 | egon      | male   |   18 |    200 |  200 | 技术         |
|  2 | alex      | female |   48 |    201 |  201 | 人力资源     |
|  3 | wupeiqi   | male   |   38 |    201 |  201 | 人力资源     |
|  4 | yuanhao   | female |   28 |    202 |  202 | 销售         |
|  5 | liwenzhou | male   |   18 |    200 |  200 | 技术         |
+----+-----------+--------+------+--------+------+--------------+
5 rows in set (0.00 sec)

　　通过对笛卡尔积的再处理，得出了两张表有对应关系的部分。但是两张表上，对方没有对上的记录并没有显示出来。因此需要使用mysql给的专用处理工具来处理。

2、内连接：只连接匹配的行

　　即取两张表共同部分，相当于上面利用条件从笛卡尔积结果中筛选出正确结果。

#department没有204这个部门，因而employee表中关于204这条员工信息没有匹配出来
mysql> select * from employee inner join department on employee.dep_id = department.id;
+----+-----------+--------+------+--------+------+--------------+
| id | name      | sex    | age  | dep_id | id   | name         |
+----+-----------+--------+------+--------+------+--------------+
|  1 | egon      | male   |   18 |    200 |  200 | 技术         |
|  2 | alex      | female |   48 |    201 |  201 | 人力资源     |
|  3 | wupeiqi   | male   |   38 |    201 |  201 | 人力资源     |
|  4 | yuanhao   | female |   28 |    202 |  202 | 销售         |
|  5 | liwenzhou | male   |   18 |    200 |  200 | 技术         |
+----+-----------+--------+------+--------+------+--------------+
5 rows in set (0.00 sec)
# 与上面笛卡尔where筛选的执行结果完全一样

 

3、外链接之左连接：在内连接的基础上保留左表的记录

　　在内连接的基础上保留左表有但右边没有的记录——以左表为准，找出所有员工信息，包括没有部门的员工。

mysql> select * from employee left join department on employee.dep_id = department.id;     
+----+------------+--------+------+--------+------+--------------+
| id | name       | sex    | age  | dep_id | id   | name         |
+----+------------+--------+------+--------+------+--------------+
|  1 | egon       | male   |   18 |    200 |  200 | 技术         |
|  5 | liwenzhou  | male   |   18 |    200 |  200 | 技术         |
|  2 | alex       | female |   48 |    201 |  201 | 人力资源     |
|  3 | wupeiqi    | male   |   38 |    201 |  201 | 人力资源     |
|  4 | yuanhao    | female |   28 |    202 |  202 | 销售         |
|  6 | jingliyang | female |   18 |    204 | NULL | NULL         |
+----+------------+--------+------+--------+------+--------------+
6 rows in set (0.00 sec)

　　没有对应的记录也显示出来了，并用NULL显示。

4、外链接之右连接：在内连接的基础上保留右表的记录

　　在内连接的基础上增加右边有左边没有的结果——以右表为准，找出所有部门信息，包括没有员工的部门。

mysql> select * from employee right join department on employee.dep_id = department.id;        
+------+-----------+--------+------+--------+------+--------------+
| id   | name      | sex    | age  | dep_id | id   | name         |
+------+-----------+--------+------+--------+------+--------------+
|    1 | egon      | male   |   18 |    200 |  200 | 技术         |
|    2 | alex      | female |   48 |    201 |  201 | 人力资源     |
|    3 | wupeiqi   | male   |   38 |    201 |  201 | 人力资源     |
|    4 | yuanhao   | female |   28 |    202 |  202 | 销售         |
|    5 | liwenzhou | male   |   18 |    200 |  200 | 技术         |
| NULL | NULL      | NULL   | NULL |   NULL |  203 | 运营         |
+------+-----------+--------+------+--------+------+--------------+
6 rows in set (0.00 sec)

　　由于employee表没有运营部门员工信息，以NULL显示。

5、全外连接：在内连接的基础上保留，左右两个表没有对应关系的记录

　　在内连接的基础上增加左边有右边没有和右边有左边没有的结果——显示全部记录

注意：mysql不支持full join

mysql> select * from employee full join department on employee.dep_id = department.id;     
ERROR 1054 (42S22): Unknown column ‘employee.dep_id‘ in ‘on clause‘

mysql使用union来间接实现全外连接

mysql> select * from employee left join department on employee.dep_id = department.id
    -> union
    -> select * from employee right join department on employee.dep_id = department.id;
+------+------------+--------+------+--------+------+--------------+
| id   | name       | sex    | age  | dep_id | id   | name         |
+------+------------+--------+------+--------+------+--------------+
|    1 | egon       | male   |   18 |    200 |  200 | 技术         |
|    5 | liwenzhou  | male   |   18 |    200 |  200 | 技术         |
|    2 | alex       | female |   48 |    201 |  201 | 人力资源     |
|    3 | wupeiqi    | male   |   38 |    201 |  201 | 人力资源     |
|    4 | yuanhao    | female |   28 |    202 |  202 | 销售         |
|    6 | jingliyang | female |   18 |    204 | NULL | NULL         |
| NULL | NULL       | NULL   | NULL |   NULL |  203 | 运营         |
+------+------------+--------+------+--------+------+--------------+
7 rows in set (0.01 sec)

　　注意union与union all的区别：union会去掉相同的记录。