MySQL DML操作--------多表联合查询实战

1. 背景

* 多表联合查询是把不同表的记录到一起的一种方式

* 在SQL标准中规划的联合(join)大致分内连接，外连接，全连接。其中外连接又分左外连接，右外连接。

2. 内连接例子 (inner join) [ 员工 --> 部门 ]　

* 查看员工表[ employees ]和部门表[ departments ]结构

mysql> desc employees;+-----------+---------------+------+-----+---------+----------------+| Field     | Type          | Null | Key | Default | Extra          |+-----------+---------------+------+-----+---------+----------------+| id        | bigint(20)    | NO   | PRI | NULL    | auto_increment || name      | varchar(64)   | NO   |     | NULL    |                || sex       | enum('M','F') | NO   |     | NULL    |                || age       | int(11)       | NO   |     | NULL    |                || depart_id | bigint(20)    | NO   |     | NULL    |                |+-----------+---------------+------+-----+---------+----------------+5 rows in set (0.00 sec)mysql> desc departments;+-------+-------------+------+-----+---------+----------------+| Field | Type        | Null | Key | Default | Extra          |+-------+-------------+------+-----+---------+----------------+| id    | bigint(20)  | NO   | PRI | NULL    | auto_increment || name  | varchar(64) | NO   |     | NULL    |                |+-------+-------------+------+-----+---------+----------------+2 rows in set (0.00 sec)

* 查看员工表[ employees ]和部门表[ departments ]数据

mysql> select * from employees;+----+-------+-----+-----+-----------+| id | name  | sex | age | depart_id |+----+-------+-----+-----+-----------+|  1 | tom   | M   |  25 |         1 ||  2 | jak   | F   |  35 |         2 ||  3 | lisea | M   |  22 |         3 |+----+-------+-----+-----+-----------+3 rows in set (0.00 sec)mysql> select * from departments;+----+------+| id | name |+----+------+|  1 | dev  ||  2 | test ||  3 | ops  |+----+------+3 rows in set (0.00 sec)

* 查询并显示所有员工id, 姓名，姓别，年龄，所在部门 (方法一)

mysql> select e.id id, e.name name, IF(e.sex = 'M', 'male', 'female') sex, e.age age, d.name    -> from employees e, departments d    -> where e.depart_id = d.id;+----+-------+--------+-----+------+| id | name  | sex    | age | name |+----+-------+--------+-----+------+|  1 | tom   | male   |  25 | dev  ||  2 | jak   | female |  35 | test ||  3 | lisea | male   |  22 | ops  |+----+-------+--------+-----+------+3 rows in set (0.03 sec)

* 查询并显示所有员工id, 姓名，姓别，年龄，所在部门 (方法二)　[ 方法一等价于方法二 ]

inner join 可以省写成 join

mysql> select e.id id, e.name name, IF(e.sex = 'M', 'male', 'female') sex, e.age age, d.name    -> from employees e inner join departments d    -> on e.depart_id = d.id;+----+-------+--------+-----+------+| id | name  | sex    | age | name |+----+-------+--------+-----+------+|  1 | tom   | male   |  25 | dev  ||  2 | jak   | female |  35 | test ||  3 | lisea | male   |  22 | ops  |+----+-------+--------+-----+------+3 rows in set (0.00 sec)

3. 外连接例子

左外连接 [ 以左表为基础，左表的全部数据，右表有的组合。右表没有的为null ]

右外连接 [ 以右表为基础，右表的全部数据，左表有的组合。左表没有的为null ]

* 查看a表和b表结构

mysql> desc a;+-------+---------+------+-----+---------+-------+| Field | Type    | Null | Key | Default | Extra |+-------+---------+------+-----+---------+-------+| data  | int(11) | YES  |     | NULL    |       |+-------+---------+------+-----+---------+-------+1 row in set (0.00 sec)mysql> desc b;+-------+---------+------+-----+---------+-------+| Field | Type    | Null | Key | Default | Extra |+-------+---------+------+-----+---------+-------+| data  | int(11) | YES  |     | NULL    |       |+-------+---------+------+-----+---------+-------+1 row in set (0.00 sec)

* 查看a表和b表数据

mysql> select * from a;+------+| data |+------+|    1 ||    2 ||    4 |+------+3 rows in set (0.00 sec)mysql> select * from b;+------+| data |+------+|    2 ||    4 ||    5 ||    6 |+------+4 rows in set (0.01 sec)

* 左外连接查询 (left join)　以a表为基础，显示a表所有数据，b表有的组合，没有显示NULLL

left outer join 可以写成 left join

mysql> select * from a left outer join b on a.data = b.data;+------+------+| data | data |+------+------+|    2 |    2 ||    4 |    4 ||    1 | NULL |+------+------+3 rows in set (0.00 sec)

* 右外连接查询 以b表为基础，显示b表所有数据，a表有的组合，没有显示NULLL

right outer join 可以写成 right join

mysql> select * from a right outer join b on a.data = b.data;+------+------+| data | data |+------+------+|    2 |    2 ||    4 |    4 || NULL |    5 || NULL |    6 |+------+------+4 rows in set (0.00 sec)

* 完全(交叉)连接查询

没有 where 子句的交叉联接将产生联接所涉及的表的笛卡尔积。

第一个表的行数乘以第二个表的行数等于笛卡尔积结果集的大小。

（a和b交叉连接产生3*4=12条记录）

mysql> select * from a corss join b;+------+------+| data | data |+------+------+|    1 |    2 ||    2 |    2 ||    4 |    2 ||    1 |    4 ||    2 |    4 ||    4 |    4 ||    1 |    5 ||    2 |    5 ||    4 |    5 ||    1 |    6 ||    2 |    6 ||    4 |    6 |+------+------+12 rows in set (0.00 sec)

4. 总结

以需求驱动技术，技术本身没有优略之分，只有业务之分。