mysql union使用

需求： 一个是帖子回复表，一个是发现回复表，web需要两个表内容显示一块，并能按照时间排序

select * from ( select 2 as table_type,id,`comment` as content,uid,c_id as post_id,add_time  from {$this->_commentTable} where uid={$params['uid']} and is_del=0 union
         select 1 as table_type,id,content,uid,i_id as post_id,add_time from {$this->_interlocutionTable}  where uid={$params['uid']} and is_del=0 and `type`=1
        ) as reply  order by add_time desc {$whereLimit}