数据库多表查询

连表查询

#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 |
+----+------------+--------+------+--------+

表的笛卡尔积

select * from 表1,表2

会直接将两张表拼接在一起,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 (0.00 sec)

内连接

inner join

select 字段 from 表1 inner join 表2 on 条件

只有两张表中条件互相匹配的项才能被显示出来

mysql> select * from employee as emp inner join department as dep on emp.dep_id = dep.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)


# 找出年龄大于25岁的员工及员工所在的部门    将内连接的表当做一个新的表,直接在后面写where条件
mysql> select emp.name,dep.name from employee as emp inner join department as dep on emp.dep_id = dep.id where age >25;
#  name字段名重复 在select时要标明表名,age字段只有一个,条件中可以直接写
+---------+--------------+
| name    | name         |
+---------+--------------+
| alex    | 人力资源     |
| wupeiqi | 人力资源     |
| yuanhao | 销售         |
+---------+--------------+
3 rows in set (0.00 sec)

# 查询两张表,并以age字段升序显示
mysql> select * from employee as emp inner join department as dep on emp.dep_id = dep.id order by age;
+----+-----------+--------+------+--------+------+--------------+
| id | name      | sex    | age  | dep_id | id   | name         |
+----+-----------+--------+------+--------+------+--------------+
|  1 | egon      | male   |   18 |    200 |  200 | 技术         |
|  5 | liwenzhou | male   |   18 |    200 |  200 | 技术         |
|  4 | yuanhao   | female |   28 |    202 |  202 | 销售         |
|  3 | wupeiqi   | male   |   38 |    201 |  201 | 人力资源     |
|  2 | alex      | female |   48 |    201 |  201 | 人力资源     |
+----+-----------+--------+------+--------+------+--------------+
5 rows in set (0.00 sec)

外链接

左外连接

left join 

select 字段 from 表1 left join 表2 on 条件

完整的显示左表中的所有数据,根据条件显示右表,没有匹配的数据显示null

mysql> select * from employee as emp left join department as dep on emp.dep_id = dep.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)

右外连接   

right join

select 字段 from 表1 right join 表2 on 条件

完整的显示右表中的所有数据,根据条件显示左表,没有匹配的数据显示null

mysql> select * from employee as emp right join department as dep on emp.dep_id = dep.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)

全外连接

union

select 字段 from 表1 left join 表2 on 条件
union
select 字段 from 表1 right join 表2 on 条件

将所有的内容都显示出来,匹配不到的内容显示null

mysql> select * from employee as emp left join department as dep on emp.dep_id = dep.id
    -> union
    -> select * from employee as emp right join department as dep on emp.dep_id = dep.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 (0.00 sec)

子查询

将一个查询语句嵌套在一个查询语句中

效率比连表查询低

# 查询人数<1的部门名字
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)

#查询技术部门的员工名字
mysql> select name from employee where dep_id = (select id from department where name='技术');
+-----------+
| name      |
+-----------+
| egon      |
| liwenzhou |
+-----------+
2 rows in set (0.00 sec)

# 查询大于所有员工平均年龄的名字和年龄
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)

连接查询和子查询一起使用

# 查询大于部门内平均年龄的名字,年龄
mysql> select * from employee as t1 inner join (select dep_id,avg(age) as age from employee group by dep_id) as t2 on t1.dep_id = t2.dep_id where t1.age > t2.age;
+----+------+--------+------+--------+--------+---------+
| id | name | sex    | age  | dep_id | dep_id | age     |
+----+------+--------+------+--------+--------+---------+
|  2 | alex | female |   48 |    201 |    201 | 43.0000 |
+----+------+--------+------+--------+--------+---------+
1 row in set (0.00 sec)