mysql - 从 Union 获得独特的值(value)

Question

场景:

Table A
MAPPING_ID    USER_TYPE    MODIFIED_ON
--------------------------------------

Table B
MAPPING_ID    ITEM_TYPE    MODIFIED_ON
--------------------------------------

我想获取最近一分钟修改过的记录[MAPPING_ID]

(SELECT mapping_id FROM A WHERE modified_on > date_sub(now(),interval 1 minute) 
UNION 
(SELECT mapping_id from B WHERE modified_on > date_sub(now(),interval 1 minute) 

但问题是，结果中可能存在重复的mapping_id!如何在 UNION 上获取distinctmapping_id？

Answer 1

UNION 默认情况下会删除重复行，除非您使用 UNION ALL，请参阅 http://dev.mysql.com/doc/refman/5.0/en/union.html

如果您想手动执行此操作:

SELECT DISTINCT * FROM (
(SELECT mapping_id FROM A WHERE modified_on > date_sub(now(),interval 1 minute) 
UNION 
(SELECT mapping_id from B WHERE modified_on > date_sub(now(),interval 1 minute) 
) subsub