有下一个查询：
select `privmsgs_id`, `contact`, `privmsgs_date`, `contentType` from (
(
select max(`privmsgs_id`) as `privmsgs_id`, `contact`, max(`privmsgs_date`) as `privmsgs_date`, `contentType` from (
(
select `privmsgs_from_userid` as `contact`, `privmsgs_id`, `privmsgs_date`, 'message' as `contentType`
from `privmsgs_table`
where `privmsgs_to_userid` = 305026
) union (
select `privmsgs_to_userid` as `contact`, `privmsgs_id`, `privmsgs_date`, 'message' as `contentType`
from `privmsgs_table`
where `privmsgs_from_userid` = 305026
)
) as `tmp` group by `contact` order by `privmsgs_date` desc
) union (
select max(`privmsgs_id`) as `privmsgs_id`, `contact`, max(`privmsgs_date`) as `privmsgs_date`, `contentType` from (
(
select `from_userid` as `contact`, `id` as `privmsgs_id`, `date` as `privmsgs_date`, 'postcard' as `contentType`
from `postcards_table`
where `to_userid` = 305026
) union (
select `to_userid` as `contact`, `id` as `privmsgs_id`, `date` as `privmsgs_date`, 'postcard' as `contentType`
from `postcards_table`
where `from_userid` = 305026
)
) as `tmp1` group by `contact` order by `privmsgs_date` desc
)
) as `rTmp` order by `privmsgs_date` desc;
有两个表tmp并tmp1通过联合合并，但字段加倍contact：
privmsgs_id contact privmsgs_date contentType
21490780 7070 1315207813 message
21556868 7070 1315215266 postcard
21226460 7754 1312025735 message
21539085 15588 1314615528 postcard
21489812 15588 1315208838 message
所以，我只需要最后一条记录（消息或明信片 - 无关紧要）和最后一条记录的 id（有问题 - 我可以在消息和明信片中分别获取 max(id)，但不能在合并表中执行此操作):
privmsgs_id contact privmsgs_date contentType
21556868 7070 1315215266 postcard
21226460 7754 1312025735 message
21489812 15588 1315208838 message
原因，为什么我不通过简化查询来做到这一点，是因为我需要特定数量的结果，所以，我只能通过一个查询来做到这一点。