MySQL实现多表查询语句介绍

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创建表

# 创建表create table department(id int,name varchar(20));create table employee1(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 employee1(name,sex,age,dep_id) values('egon','male',18,200),('alex','female',48,201),('tom','male',38,201),('yuanhao','female',28,202),('lidawei','male',18,200),('jinkezhou','female',18,204);# 查看表mysql> select * from employee1;+----+-----------+--------+------+--------+| id | name      | sex    | age  | dep_id |+----+-----------+--------+------+--------+|  1 | egon      | male   |   18 |    200 ||  2 | alex      | female |   48 |    201 ||  3 | tom       | male   |   38 |    201 ||  4 | yuanhao   | female |   28 |    202 ||  5 | lidawei   | male   |   18 |    200 ||  6 | jinkezhou | female |   18 |    204 |+----+-----------+--------+------+--------+6 rows in set (0.00 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)

多表连接查询

交叉连接

交叉连接：不适用任何匹配条件。生成笛卡尔积

mysql> select * from employee1 ,department;

内连接

内连接：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。（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 * from employee1,department where employee1.dep_id=department.id;#上面用where表示的可以用下面的内连接表示，建议使用下面的那种方法mysql> select * from employee1 inner join department on employee1.dep_id=department.id;# 也可以这样表示哈mysql> select employee1.id,employee1.name,employee1.age,employee1.sex,department.name from employee1,department where employee1.dep_id=department.id;

左连接left

优先显示左表全部记录。

#左链接：在按照on的条件取到两张表共同部分的基础上，保留左表的记录mysql> select * from employee1 left join department on department.id=employee1.dep_id;mysql> select * from department left join  employee1 on department.id=employee1.dep_id;

右连接right

优先显示右表全部记录。

#右链接：在按照on的条件取到两张表共同部分的基础上，保留右表的记录mysql> select * from employee1 right join department on department.id=employee1.dep_id;mysql> select * from department right join employee1 on department.id=employee1.dep_id;

全部连接join

mysql> select * from department full join employee1;

符合条件多表查询

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,
即找出公司所有部门中年龄大于25岁的员工

mysql> select * from employee1 inner join department on employee1.dep_id=department.id and 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

mysql> select * from employee1 inner join department on employee1.dep_id=department.id and age>25 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

示例：

# 查询平均年龄在25岁以上的部门名mysql> select name from department where id in ( select dep_id from employee1 group by dep_id having avg(age) > 25 );# 查看技术部员工姓名mysql> select name from employee1 where dep_id = (select id from department where name='技术');# 查看小于2人的部门名mysql> select name from department where id in (select dep_id from employee1 group by dep_id having count(id) < 2) union select name from department where id not in (select distinct dep_id from employee1);# 提取空部门                              #有人的部门mysql> select * from department where id not in (select distinct dep_id from employee1);

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