| 一、介绍 |
本节主题：
- 多表连接查询
- 复合条件连接查询
- 子查询
准备表：
#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 | +----+------------+--------+------+--------+
需要注意的是在employee表有一个dep_id=204是department表没有的。同时，department表有一个id=203在employee表没有对应的记录。
| 二、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 | 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 (0.01 sec)
每个员工和四个部门做一次对应关系。但这四次匹配仅有一次是需要的：即dep_id和id相同的那个。
mysql> select * from employee, department where employee.dep_id = department.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)
通过对笛卡尔积的再处理，得出了两张表有对应关系的部分。但是两张表上，对方没有对上的记录并没有显示出来。因此需要使用mysql给的专用处理工具来处理。
2、内连接：只连接匹配的行
即取两张表共同部分，相当于上面利用条件从笛卡尔积结果中筛选出正确结果。
#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 employee inner join department on employee.dep_id = department.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) # 与上面笛卡尔where筛选的执行结果完全一样
3、外链接之左连接：在内连接的基础上保留左表的记录
在内连接的基础上保留左表有但右边没有的记录——以左表为准，找出所有员工信息，包括没有部门的员工。
mysql> select * from employee left 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 | +----+------------+--------+------+--------+------+--------------+ 6 rows in set (0.00 sec)
没有对应的记录也显示出来了，并用NULL显示。
4、外链接之右连接：在内连接的基础上保留右表的记录
在内连接的基础上增加右边有左边没有的结果——以右表为准，找出所有部门信息，包括没有员工的部门。
mysql> 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 | | 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)
由于employee表没有运营部门员工信息，以NULL显示。
5、全外连接：在内连接的基础上保留，左右两个表没有对应关系的记录
在内连接的基础上增加左边有右边没有和右边有左边没有的结果——显示全部记录
注意：mysql不支持full join
mysql> select * from employee full join department on employee.dep_id = department.id; ERROR 1054 (42S22): Unknown column 'employee.dep_id' in 'on clause'
mysql使用union来间接实现全外连接
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 (0.01 sec)
注意union与union all的区别：union会去掉相同的记录。