MySQL多表查询

mysql多表查询

建表与数据准备

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

create table employee(      #员工表
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
('daxia','female',18,200),
('anwen','male',23,201),
('wudi','male',38,201),
('yage','female',23,202),
('dafei','male',18,200),
('jingyu','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 | daxia | female | 18 | 200 |
| 2 | anwen | male | 23 | 201 |
| 3 | wudi | male | 38 | 201 |
| 4 | yage | female | 23 | 202 |
| 5 | dafei | male | 18 | 200 |
| 6 | jingyu | female | 18 | 204 |
+----+------------+--------+------+--------+

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 | daxia       | female   |   18 |    200 |  200 | 技术         |
|  1 | daxia       | female   |   18 |    200 |  201 | 人力资源     |
|  1 | daxia       | female   |   18 |    200 |  202 | 销售         |
|  1 | daxia       | female   |   18 |    200 |  203 | 运营         |
|  2 | anwen       | male     |   48 |    201 |  200 | 技术         |
|  2 | anwen       | male     |   48 |    201 |  201 | 人力资源     |
|  2 | anwen       | male     |   48 |    201 |  202 | 销售         |
|  2 | anwen       | male     |   48 |    201 |  203 | 运营         |
|  3 | wudi        | male   |   38 |    201 |  200 | 技术         |
|  3 | wudi        | male   |   38 |    201 |  201 | 人力资源     |
|  3 | wudi        | male   |   38 |    201 |  202 | 销售         |
|  3 | wudi        | male   |   38 |    201 |  203 | 运营         |
|  4 | yage        | female |   28 |    202 |  200 | 技术         |
|  4 | yage        | female |   28 |    202 |  201 | 人力资源     |
|  4 | yage        | female |   28 |    202 |  202 | 销售         |
|  4 | yage        | female |   28 |    202 |  203 | 运营         |
|  5 | dafei       | male   |   18 |    200 |  200 | 技术         |
|  5 | dafei       | male   |   18 |    200 |  201 | 人力资源     |
|  5 | dafei       | male   |   18 |    200 |  202 | 销售         |
|  5 | dafei       | male   |   18 |    200 |  203 | 运营         |
|  6 | jingyu      | female |   18 |    204 |  200 | 技术         |
|  6 | jingyu      | female |   18 |    204 |  201 | 人力资源     |
|  6 | jingyu      | female |   18 |    204 |  202 | 销售         |
|  6 | jingyu      | female |   18 |    204 |  203 | 运营         |
+----+------------+--------+------+--------+------+--------------+

2 内连接：只连接匹配的行

#找两张表共有的部分，相当于利用条件从笛卡尔积结果中筛选出了正确的结果
#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
mysql> select employee.id,employee.name,employee.age,employee.sex,department.name from employee inner join department on employee.dep_id=department.id; 
+----+-----------+------+--------+--------------+
| id | name      | age  | sex    | name         |
+----+-----------+------+--------+--------------+
|  1 | daxia      |   18 | female   | 技术       |
|  2 | anwen      |   48 | male     | 人力资源    |
|  3 | wudi       |   38 | male     | 人力资源    |
|  4 | yage       |   28 | female   | 销售       |
|  5 | dafei      |   18 | male     | 技术       |
+----+-----------+------+--------+--------------+

#上述sql等同于
mysql> select employee.id,employee.name,employee.age,employee.sex,department.name from employee,department where employee.dep_id=department.id;

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 | daxia       | 技术         |
|  5 | dafei       | 技术         |
|  2 | anwen       | 人力资源     |
|  3 | wudi        | 人力资源     |
|  4 | dafei       | 销售         |
|  6 | jingyu      | null         |
+----+------------+--------------+

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 | daxia      | 技术         |
|    2 | anwen      | 人力资源     |
|    3 | wudi       | 人力资源     |
|    4 | yage      | 销售         |
|    5 | dafei      | 技术         |
| null | null       | 运营         |
+------+-----------+--------------+

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
#注意：mysql不支持全外连接 full join
#强调：mysql可以使用此种方式间接实现全外连接
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
;
#查看结果
+------+------------+--------+------+--------+------+--------------+
| id   | name       | sex    | age  | dep_id | id   | name         |
+------+------------+--------+------+--------+------+--------------+
|    1 | daxia       | female   |   18 |    200 |  200 | 技术         |
|    5 | dafei       | male   |   18 |    200 |  200 | 技术         |
|    2 | anwen       | male |   48 |    201 |  201 | 人力资源     |
|    3 | wudi        | male   |   38 |    201 |  201 | 人力资源     |
|    4 | yage        | female |   28 |    202 |  202 | 销售         |
|    6 | jingyu      | female |   18 |    204 | null | null        |
| null | null        | null   | null |   null |  203 | 运营         |
+------+------------+--------+------+--------+------+--------------+

#注意 union与union all的区别：union会去掉相同的纪录

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
select employee.name,department.name from employee inner join department
    on employee.dep_id = department.id
    where age > 25;

#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
select employee.id,employee.name,employee.age,department.name from employee,department
    where employee.dep_id = department.id
    and age > 25
    order by age asc;

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
select id,name from department
    where id in 
        (select dep_id from employee group by dep_id having avg(age) > 25);

#查看技术部员工姓名
select name from employee
    where dep_id in 
        (select id from department where name='技术');

#查看不足1人的部门名(子查询得到的是有人的部门id)
select name from department where id not in (select distinct dep_id from employee);

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 emp where age > (select avg(age) from emp);
+---------+------+
| name    | age  |
+---------+------+
| anwen    | 48   |
| wudi     | 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
select t1.name,t1.age from emp t1
inner join 
(select dep_id,avg(age) avg_age from emp group by dep_id) t2
on t1.dep_id = t2.dep_id
where t1.age > t2.avg_age; 

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

#department表中存在dept_id=203，ture
mysql> select * from employee
    ->     where exists
    ->         (select id from department where id=200);
+----+------------+--------+------+--------+
| id | name       | sex    | age  | dep_id |
+----+------------+--------+------+--------+
|  1 | daxia       | female   |   18 |    200 |
|  2 | anwen       | male |   48 |    201 |
|  3 | wudi        | male   |   38 |    201 |
|  4 | yage        | female |   28 |    202 |
|  5 | dafei       | male   |   18 |    200 |
|  6 | jingyu      | female |   18 |    204 |
+----+------------+--------+------+--------+

#department表中存在dept_id=205，false
mysql> select * from employee
    ->     where exists
    ->         (select id from department where id=204);
empty set (0.00 sec)