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本文实例讲述了MySQL多表查询。分享给大家供大家参考，具体如下：
准备工作：准备两张表，部门表（department）、员工表（employee）
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 | | +-------+-------------+------+-----+---------+-------+ 2 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 | |
+--------+-----------------------+------+-----+---------+----------------+
5 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 | +------+--------------+ 4 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 | +----+----------+--------+------+--------+ 6 rows in set (0.00 sec)
ps:观察两张表，发现department表中id=203部门在employee中没有对应的员工，发现employee中id=6的员工在department表中没有对应关系。
一多表链接查询
SELECT 字段列表
FROM 表1 INNER|LEFT|RIGHT JOIN 表2
ON 表1.字段 = 表2.字段;
（1）先看第一种情况交叉连接：不适用任何匹配条件。生成笛卡尔积.--->重复最多
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 | 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 |
（2）内连接：只连接匹配的行，以双方为基准
#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 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 | 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 | nvshen | 18 | male | 技术 | +----+---------+------+--------+--------------+ 5 rows in set (0.00 sec)
#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 | egon | 技术 | | 5 | nvshen | 技术 | | 2 | alex | 人力资源 | | 3 | wupeiqi | 人力资源 | | 4 | yuanhao | 销售 | | 6 | xiaomage | 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 | nvshen | 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(了解)
#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相关阅读 >>
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