1. 背景
* 多表聯合查詢是把不同表的記錄到一起的一種方式
* 在SQL標準中規劃的聯合(join)大致分內連接,外連接,全連接。其中外連接又分左外連接,右外連接。
2. 內連接例子 (inner join) [ 員工 --> 部門 ]
* 查看員工表[ employees ]和部門表[ departments ]結構
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)
* 查看員工表[ employees ]和部門表[ departments ]數據
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)
* 查詢並顯示所有員工id, 姓名,姓別,年齡,所在部門 (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)
* 查詢並顯示所有員工id, 姓名,姓別,年齡,所在部門 (方法二) [ 方法一等價於方法二 ]
inner join 可以省寫成 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. 外連接例子
左外連接 [ 以左表爲基礎,左表的全部數據,右表有的組合。右表沒有的爲null ]
右外連接 [ 以右表爲基礎,右表的全部數據,左表有的組合。左表沒有的爲null ]
* 查看a表和b表結構
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表數據
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)
* 左外連接查詢 (left join) 以a表爲基礎,顯示a表所有數據,b表有的組合,沒有顯示NULLL
left outer join 可以寫成 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)
* 右外連接查詢 以b表爲基礎,顯示b表所有數據,a表有的組合,沒有顯示NULLL
right outer join 可以寫成 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)連接查詢
沒有 where 子句的交叉聯接將產生聯接所涉及的表的笛卡爾積。
第一個表的行數乘以第二個表的行數等於笛卡爾積結果集的大小。
(a和b交叉連接產生3*4=12條記錄)
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. 總結
以需求驅動技術,技術本身沒有優略之分,只有業務之分。