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8.4.３

多表查询

多表连接查询
复合条件连接查询
子查询

准备工作

create database db6 charset utf8;
use db6;
#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
('cmz','male',18,200),
('leco','female',48,201),
('loocha','male',38,201),
('odes','female',28,202),
('huawei','male',18,200),
('icpc','female',18,204);

mysql> create database db6 charset utf8;
Query OK, 1 row affected (0.00 sec)

mysql> use db6;
Database changed

mysql> create table department(
    -> id int,
    -> name varchar(20) 
    -> );
Query OK, 0 rows affected (0.03 sec)
 
mysql> 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
    -> );
Query OK, 0 rows affected (0.03 sec)

mysql> insert into department values
    -> (200,'技术'),
    -> (201,'人力资源'),
    -> (202,'销售'),
    -> (203,'运营');
Query OK, 4 rows affected (0.00 sec)
Records: 4  Duplicates: 0  Warnings: 0

mysql> 
mysql> insert into employee(name,sex,age,dep_id) values
    -> ('cmz','male',18,200),
    -> ('leco','female',48,201),
    -> ('loocha','male',38,201),
    -> ('odes','female',28,202),
    -> ('huawei','male',18,200),
    -> ('icpc','female',18,204)
    -> ;
Query OK, 6 rows affected (0.01 sec)
Records: 6  Duplicates: 0  Warnings: 0

操作过程

mysql> select * from department;
+------+--------------+
| id   | name         |
+------+--------------+
|  200 | 技术         |
|  201 | 人力资源     |
|  202 | 销售         |
|  203 | 运营         |
+------+--------------+
4 rows in set (0.00 sec)

mysql> select * from employee;
+----+--------+--------+------+--------+
| id | name   | sex    | age  | dep_id |
+----+--------+--------+------+--------+
|  1 | cmz    | male   |   18 |    200 |
|  2 | leco   | female |   48 |    201 |
|  3 | loocha | male   |   38 |    201 |
|  4 | odes   | female |   28 |    202 |
|  5 | huawei | male   |   18 |    200 |
|  6 | icpc   | female |   18 |    204 |
+----+--------+--------+------+--------+
6 rows in set (0.00 sec)

mysql> desc department;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id    | int(11)     | YES  |     | NULL    |       |
| name  | varchar(20) | YES  |     | NULL    |       |
+-------+-------------+------+-----+---------+-------+
2 rows in set (0.00 sec)

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    |                |
+--------+-----------------------+------+-----+---------+----------------+
5 rows in set (0.00 sec)

表结构

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 | cmz    | male   |   18 |    200 |  200 | 技术         |
|  1 | cmz    | male   |   18 |    200 |  201 | 人力资源     |
|  1 | cmz    | male   |   18 |    200 |  202 | 销售         |
|  1 | cmz    | male   |   18 |    200 |  203 | 运营         |
|  2 | leco   | female |   48 |    201 |  200 | 技术         |
|  2 | leco   | female |   48 |    201 |  201 | 人力资源     |
|  2 | leco   | female |   48 |    201 |  202 | 销售         |
|  2 | leco   | female |   48 |    201 |  203 | 运营         |
|  3 | loocha | male   |   38 |    201 |  200 | 技术         |
|  3 | loocha | male   |   38 |    201 |  201 | 人力资源     |
|  3 | loocha | male   |   38 |    201 |  202 | 销售         |
|  3 | loocha | male   |   38 |    201 |  203 | 运营         |
|  4 | odes   | female |   28 |    202 |  200 | 技术         |
|  4 | odes   | female |   28 |    202 |  201 | 人力资源     |
|  4 | odes   | female |   28 |    202 |  202 | 销售         |
|  4 | odes   | female |   28 |    202 |  203 | 运营         |
|  5 | huawei | male   |   18 |    200 |  200 | 技术         |
|  5 | huawei | male   |   18 |    200 |  201 | 人力资源     |
|  5 | huawei | male   |   18 |    200 |  202 | 销售         |
|  5 | huawei | male   |   18 |    200 |  203 | 运营         |
|  6 | icpc   | female |   18 |    204 |  200 | 技术         |
|  6 | icpc   | female |   18 |    204 |  201 | 人力资源     |
|  6 | icpc   | female |   18 |    204 |  202 | 销售         |
|  6 | icpc   | female |   18 |    204 |  203 | 运营         |
+----+--------+--------+------+--------+------+--------------+
24 rows in set (0.01 sec)

单纯的笛卡尔级对应关系没有太多意义。

其实我门是想取出如下关系，employee.dep_id = department.id;

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

我们有专门的内连接。

mysql> select * from employee inner join department on employee.dep_id = department.id;
+----+--------+--------+------+--------+------+--------------+
| id | name   | sex    | age  | dep_id | id   | name         |
+----+--------+--------+------+--------+------+--------------+
|  1 | cmz    | male   |   18 |    200 |  200 | 技术         |
|  2 | leco   | female |   48 |    201 |  201 | 人力资源     |
|  3 | loocha | male   |   38 |    201 |  201 | 人力资源     |
|  4 | odes   | female |   28 |    202 |  202 | 销售         |
|  5 | huawei | male   |   18 |    200 |  200 | 技术         |
+----+--------+--------+------+--------+------+--------------+
5 rows in set (0.00 sec)

连表操作
１．内连接，只取两张表的共同部分
２．左连接 在内连接的基础上保留左表的记录
３．右连接 在内连接的基础上保留右边表的记录
４．全外连接 在内连接的基础上保留左右边没有对应关系的记录（左右连接的去重）

连表操作
１．内连接，只取两张表的共同部分
select * from employee inner join department on employee.dep_id = department.id;
mysql> select * from employee inner join department on employee.dep_id = department.id;
+----+--------+--------+------+--------+------+--------------+
| id | name   | sex    | age  | dep_id | id   | name         |
+----+--------+--------+------+--------+------+--------------+
|  1 | cmz    | male   |   18 |    200 |  200 | 技术         |
|  2 | leco   | female |   48 |    201 |  201 | 人力资源     |
|  3 | loocha | male   |   38 |    201 |  201 | 人力资源     |
|  4 | odes   | female |   28 |    202 |  202 | 销售         |
|  5 | huawei | male   |   18 |    200 |  200 | 技术         |
+----+--------+--------+------+--------+------+--------------+
5 rows in set (0.00 sec)
２．左连接 在内连接的基础上保留左表的记录
select * from employee left join department on employee.dep_id = department.id;
mysql> select * from employee left join department on employee.dep_id = department.id;
+----+--------+--------+------+--------+------+--------------+
| id | name   | sex    | age  | dep_id | id   | name         |
+----+--------+--------+------+--------+------+--------------+
|  1 | cmz    | male   |   18 |    200 |  200 | 技术         |
|  5 | huawei | male   |   18 |    200 |  200 | 技术         |
|  2 | leco   | female |   48 |    201 |  201 | 人力资源     |
|  3 | loocha | male   |   38 |    201 |  201 | 人力资源     |
|  4 | odes   | female |   28 |    202 |  202 | 销售         |
|  6 | icpc   | female |   18 |    204 | NULL | NULL         |
+----+--------+--------+------+--------+------+--------------+
6 rows in set (0.01 sec)

３．右连接 在内连接的基础上保留右边表的记录
select * from employee right join department on employee.dep_id = department.id;
mysql> select * from employee right join department on employee.dep_id = department.id;
+------+--------+--------+------+--------+------+--------------+
| id   | name   | sex    | age  | dep_id | id   | name         |
+------+--------+--------+------+--------+------+--------------+
|    1 | cmz    | male   |   18 |    200 |  200 | 技术         |
|    2 | leco   | female |   48 |    201 |  201 | 人力资源     |
|    3 | loocha | male   |   38 |    201 |  201 | 人力资源     |
|    4 | odes   | female |   28 |    202 |  202 | 销售         |
|    5 | huawei | male   |   18 |    200 |  200 | 技术         |
| NULL | NULL   | NULL   | NULL |   NULL |  203 | 运营         |
+------+--------+--------+------+--------+------+--------------+
6 rows in set (0.00 sec)

４．全外连接 在内连接的基础上保留左右边没有对应关系的记录（左右连接的去重）

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;

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 | cmz    | male   |   18 |    200 |  200 | 技术         |
|    5 | huawei | male   |   18 |    200 |  200 | 技术         |
|    2 | leco   | female |   48 |    201 |  201 | 人力资源     |
|    3 | loocha | male   |   38 |    201 |  201 | 人力资源     |
|    4 | odes   | female |   28 |    202 |  202 | 销售         |
|    6 | icpc   | female |   18 |    204 | NULL | NULL         |
| NULL | NULL   | NULL   | NULL |   NULL |  203 | 运营         |
+------+--------+--------+------+--------+------+--------------+
7 rows in set (0.00 sec)

小练习

＃查询平均年龄大于３０的部门名
select department.name,avg(age) from employee inner join department on employee.dep_id = department.id
  group by department.name
  having avg(age)>30;

#操作过程
mysql> select department.name,avg(age) from employee inner join department on employee.dep_id = department.id
    ->   group by department.name
    ->   having avg(age)>30;
+--------------+----------+
| name         | avg(age) |
+--------------+----------+
| 人力资源     |  43.0000 |
+--------------+----------+
1 row in set (0.00 sec)

查看全文

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原文地址：https://www.cnblogs.com/caimengzhi/p/8574116.html

8.4.３

二 多表连接查询

二多表连接查询