数据库开发——MySQL——多表查询

一、准备表

创建表：

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);

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    |       |
+-------+-------------+------+-----+---------+-------+
2 rows in set (1.65 sec)

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> 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)

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

二、多表连接查询

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 (0.34 sec)

2 内连接：只连接匹配的行

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

3 外链接之左连接：优先显示左表全部记录

以左表为准，即找出所有员工信息，当然包括没有部门的员工。

本质就是：在内连接的基础上增加左边有右边没有的结果。

mysql> select employee.id,employee.name,department.name as depart_name from employee left join department on employee.dep_id=department.id;
+----+------------+--------------+
| id | name       | depart_name  |
+----+------------+--------------+
|  1 | egon       | 技术         |
|  5 | liwenzhou  | 技术         |
|  2 | alex       | 人力资源     |
|  3 | wupeiqi    | 人力资源     |
|  4 | yuanhao    | 销售         |
|  6 | jingliyang | NULL         |
+----+------------+--------------+
6 rows in set (0.00 sec)

4 外链接之右连接：优先显示右表全部记录

以右表为准，即找出所有部门信息，包括没有员工的部门。

本质就是：在内连接的基础上增加右边有左边没有的结果。

mysql> select employee.id,employee.name,department.name as depart_name from employee right join department on employee.dep_id=department.id;
+------+-----------+--------------+
| id   | name      | depart_name  |
+------+-----------+--------------+
|    1 | egon      | 技术         |
|    2 | alex      | 人力资源     |
|    3 | wupeiqi   | 人力资源     |
|    4 | yuanhao   | 销售         |
|    5 | liwenzhou | 技术         |
| NULL | NULL      | 运营         |
+------+-----------+--------------+
6 rows in set (0.00 sec)

5 全外连接：显示左右两个表全部记录

全外连接：在内连接的基础上增加左边有右边没有的和右边有左边没有的结果。

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 (0.04 sec)

注意 union与union all的区别：union会去掉相同的纪录

三、符合条件连接查询

示例1：以内连接的方式查询employee和department表，并且employee表中的age字段值必须大于25,即找出年龄大于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 (0.10 sec)

示例2：以内连接的方式查询employee和department表，并且以age字段的升序方式显示

mysql> select employee.name, employee.age, department.name from employee inner join department on employee.dep_id = department.id order by age asc;
+-----------+------+--------------+
| name      | age  | name         |
+-----------+------+--------------+
| egon      |   18 | 技术         |
| liwenzhou |   18 | 技术         |
| yuanhao   |   28 | 销售         |
| wupeiqi   |   38 | 人力资源     |
| alex      |   48 | 人力资源     |
+-----------+------+--------------+
5 rows in set (1.81 sec)

四、子查询

1：子查询是将一个查询语句嵌套在另一个查询语句中。
2：内层查询语句的查询结果，可以为外层查询语句提供查询条件。
3：子查询中可以包含：IN、NOT IN、ANY、ALL、EXISTS 和 NOT EXISTS等关键字
4：还可以包含比较运算符：= 、 !=、> 、<等

1 带IN关键字的子查询

查询平均年龄在25岁以上的部门名

mysql> select id, name 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 (0.11 sec)

查看技术部员工姓名

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

2 带比较运算符的子查询

比较运算符：=、!=、>、>=、<、<=、<>

查询大于所有人平均年龄的员工名与年龄

mysql> select name, age from employee where age > (select avg(age) from employee);
+---------+------+
| name    | age  |
+---------+------+
| alex    |   48 |
| wupeiqi |   38 |
+---------+------+
2 rows in set (0.00 sec)

查询大于部门内平均年龄的员工名、年龄

mysql> select name, age from employee as t1 inner join (select dep_id, avg(age) as avg_age from employee group by dep_id) as t2 on t1.dep_id = t2.dep_id where t1.age > t2.avg_age;
+------+------+
| name | age  |
+------+------+
| alex |   48 |
+------+------+
1 row in set (0.00 sec)

3 带EXISTS关键字的子查询

EXISTS关字键字表示存在。

在使用EXISTS关键字时，内层查询语句不返回查询的记录。

而是返回一个真假值，True或False

当返回True时，外层查询语句将进行查询；当返回值为False时，外层查询语句不进行查询。

mysql> select * from employee where exists (select id from department where id=200);	# id存在
+----+------------+--------+------+--------+
| 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 |
+----+------------+--------+------+--------+
6 rows in set (0.10 sec)

mysql> select * from employee where exists (select id from department where id=204);	# id不存在
Empty set (0.00 sec)