对union使用order by或limit

  如果您想使用ORDER BY或LIMIT子句来对全部UNION结果进行分类或限制,则应对单个地SELECT语句加圆括号,并把ORDER BY或LIMIT放到最后一个的后面。
例如:

(SELECT a FROM tbl_name WHERE a=10 AND B=1)   
    UNION 
(SELECT a FROM tbl_name WHERE a=11 AND B=2) 
    ORDER BY a LIMIT 10; 

麻烦一点也可以这么干:

select userid from 
    (select userid from testa 
        union all 
    select userid from testb) t  
order by userid limit 0,1; 
-- 如果你还想group by,而且还有条件,那么:
select userid from 
    (select userid from testa
         union all 
    select userid from testb) t 
group by userid having count(userid) = 2; 
--注意:from的表必须有名字,如果是子查询就要有别名

#区分多表

专门做一个字段,即可区分别名。例如:

select posts_id,posts_name,'users' as table_name from yy_posts
UNION
select user_id,user_nickname,'posts' as table_name from yy_user