mysql实现full outer join

由于mysql设计时不支持full outer join,

所以如果有全连接需求时，需要一点小技巧来实现。

full outer join = left outer join union all right outer join

看如下表结构：

A:

+------+----------+

| id | username |

+------+----------+

| 1 | wuwh |

| 2 | meimei |

+------+----------+

B:

+------+------------+

| id | objectname |

+------+------------+

| 1 | 电子 |

| 3 | 衣服 |

+------+------------+

C:

+------+--------------+

| id | detail |

+------+--------------+

| 1 | 电子详情 |

| 3 | 衣服详情 |

| 4 | 车子详情 |

+------+--------------+

先实现一个小需求：

A full outer join B之后，合并两列id，并且id不能重复,sql如下：

select distinct aaabbb.aaabbb_id,username,objectname from(

SELECT

CASE

WHEN aaa.id IS NOT NULL THEN aaa.id

WHEN bbb.id IS NOT NULL THEN bbb.id

END AS aaabbb_id,

username, objectname

FROM aaa left join bbb on aaa.id = bbb.id

union all

SELECT

CASE

WHEN aaa.id IS NOT NULL THEN aaa.id

WHEN bbb.id IS NOT NULL THEN bbb.id

END AS aaabbb_id,

username, objectname

FROM aaa right join bbb on aaa.id = bbb.id;

)ccc

结果：

+-----------+----------+------------+

| aaabbb_id | username | objectname |

+-----------+----------+------------+

| 1 | wuwh | 电子 |

| 2 | meimei | NULL |

| 3 | NULL | 衣服 |

+-----------+----------+------------+

接下来是三路full outer join

SELECT DISTINCT com_id

, username

, objectname

, detail

FROM (

SELECT IF(aaabbb.aaabbb_id IS NOT NULL, aaabbb.aaabbb_id, IF(ccc.id IS NOT NULL, ccc.id, '')) AS com_id

, username

, objectname

, detail

FROM (

SELECT CASE

WHEN aaa.id IS NOT NULL THEN aaa.id

WHEN bbb.id IS NOT NULL THEN bbb.id

END AS aaabbb_id

, username

, objectname

FROM aaa

LEFT OUTER JOIN bbb

ON aaa.id = bbb.id

UNION ALL

SELECT CASE

WHEN aaa.id IS NOT NULL THEN aaa.id

WHEN bbb.id IS NOT NULL THEN bbb.id

END AS aaabbb_id

, username

, objectname

FROM aaa

RIGHT OUTER JOIN bbb

ON aaa.id = bbb.id

) aaabbb

LEFT OUTER JOIN ccc

ON aaabbb.aaabbb_id = ccc.id

UNION ALL

SELECT IF(aaabbb.aaabbb_id IS NOT NULL, aaabbb.aaabbb_id, IF(ccc.id IS NOT NULL, ccc.id, '')) AS com_id

, username

, objectname

, detail

FROM (

SELECT CASE

WHEN aaa.id IS NOT NULL THEN aaa.id

WHEN bbb.id IS NOT NULL THEN bbb.id

END AS aaabbb_id

, username

, objectname

FROM aaa

LEFT OUTER JOIN bbb

ON aaa.id = bbb.id

UNION ALL

SELECT CASE

WHEN aaa.id IS NOT NULL THEN aaa.id

WHEN bbb.id IS NOT NULL THEN bbb.id

END AS aaabbb_id

, username

, objectname

FROM aaa

RIGHT OUTER JOIN bbb

ON aaa.id = bbb.id

) aaabbb

RIGHT OUTER JOIN ccc

ON aaabbb.aaabbb_id = ccc.id

) ddd;

结果：

+--------+----------+------------+--------------+

| com_id | username | objectname | detail |

+--------+----------+------------+--------------+

| 1 | wuwh | 电子 | 电子详情 |

| 3 | NULL | 衣服 | 衣服详情 |

| 2 | meimei | NULL | NULL |

| 4 | NULL | NULL | 车子详情 |

+--------+----------+------------+--------------+