MySql

多表查询

• 1. 多表连接查询
• 2.符合条件连接查询
• 3. 子查询

数据准备

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),
('nvshen','male',18,200),
('xiaomage','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    |       |
+-------+-------------+------+-----+---------+-------+
rows in set (0.19 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    |                |
+--------+-----------------------+------+-----+---------+----------------+
rows in set (0.01 sec)

mysql> select * from department;
+------+--------------+
| id   | name         |
+------+--------------+
|  200 | 技术         |
|  201 | 人力资源     |
|  202 | 销售         |
|  203 | 运营         |
+------+--------------+
rows in set (0.02 sec)

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 | nvshen   | male   |   18 |    200 |
|  6 | xiaomage | female |   18 |    204 |
+----+----------+--------+------+--------+
rows in set (0.00 sec)

1. 多表连接查询

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;

1.1 一个概念 : 笛卡尔积

• 笛卡尔积

  mysql> select * from employee2,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 | nvshen   | male   |   18 |    200 |  200 | 技术         |
  |  5 | nvshen   | male   |   18 |    200 |  201 | 人力资源     |
  |  5 | nvshen   | male   |   18 |    200 |  202 | 销售         |
  |  5 | nvshen   | male   |   18 |    200 |  203 | 运营         |
  |  6 | xiaomage | female |   18 |    204 |  200 | 技术         |
  |  6 | xiaomage | female |   18 |    204 |  201 | 人力资源     |
  |  6 | xiaomage | female |   18 |    204 |  202 | 销售         |
  |  6 | xiaomage | female |   18 |    204 |  203 | 运营         |
  +----+----------+--------+------+--------+------+--------------+
  24 rows in set (0.11 sec)

• 符合条件查询

  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 | nvshen  | male   |   18 |    200 |  200 | 技术         |
  +----+---------+--------+------+--------+------+--------------+
  5 rows in set (0.01 sec)

1.2 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 | 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 | nvshen  | male   |   18 |    200 |  200 | 技术         |
+----+---------+--------+------+--------+------+--------------+
5 rows in set (0.00 sec)

通过上表可以看出,内连接是找到两张表共有的部分,相当于利用条件从笛卡尔积结果中筛选出了匹配的结果

(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) -> 与上面的符合条件查询相同

1.3 左连接或右连接 (优先显示左表或者右表的全部记录)

• 左连接

  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 | nvshen   | 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 | xiaomage | female |   18 |    204 | NULL | NULL         |
  +----+----------+--------+------+--------+------+--------------+
  6 rows in set (0.00 sec)

• 右连接

  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 | nvshen  | male   |   18 |    200 |  200 | 技术         |
  | NULL | NULL    | NULL   | NULL |   NULL |  203 | 运营         |
  +------+---------+--------+------+--------+------+--------------+
  6 rows in set (0.00 sec)

1.4 全外连接  (显示两个表中的全部记录)

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 | nvshen   | 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 | xiaomage | female |   18 |    204 | NULL | NULL         |
| NULL | NULL     | NULL   | NULL |   NULL |  203 | 运营         |
+------+----------+--------+------+--------+------+--------------+
7 rows in set (0.01 sec)

2. 符合条件连接查询

• 找出年龄大于25岁的员工以及员工所在的部门

  mysql> select employee.name,department.name from employee inner join department on employee.dep_id = department.id where age>25;
  +---------+--------------+
  | name    | name         |
  +---------+--------------+
  | alex    | 人力资源     |
  | wupeiqi | 人力资源     |
  | yuanhao | 销售         |
  +---------+--------------+
  3 rows in set (0.00 sec)

• 找出年龄大于25岁的员工以及员工所在的部门,并且以age字段的升序方式显示。

  mysql> select employee.name,department.name from employee inner join department on employee.dep_id = department.id where age>25 order by age asc;
  +---------+--------------+
  | name    | name         |
  +---------+--------------+
  | yuanhao | 销售         |
  | wupeiqi | 人力资源     |
  | alex    | 人力资源     |
  +---------+--------------+
  3 rows in set (0.00 sec)

3. 子查询

3.1 带in 关键字的子查询(练习题)

• 查询平均年龄在25岁以上的部门的名称

  mysql> select dep_id from employee group by dep_id having avg(age)>25;
  +--------+
  | dep_id |
  +--------+
  |    201 |
  |    202 |
  +--------+
  2 rows in set (0.38 sec)
  
  mysql> select * from department where id in (select dep_id from employee group by dep_id having avg(age)>25);
  +------+--------------+
  | id   | name         |
  +------+--------------+
  |  201 | 人力资源     |
  |  202 | 销售         |
  +------+--------------+
  2 rows in set (0.00 sec)

• 查看技术部员工的姓名

  mysql> select name from employee where dep_id in(select id from department where name ="技术");
  +--------+
  | name   |
  +--------+
  | egon   |
  | nvshen |
  +--------+
  2 rows in set (0.00 sec)

• 查看不足一人的部门名

  mysql> select name from department where id not  in(select dep_id from employee group by dep_id );
  +--------+
  | name   |
  +--------+
  | 运营   |
  +--------+
  1 row in set (0.00 sec

3.2 带比较运算符的子查询