如何合併兩個生成的表使用由'a'字段的組由'b'字段的最大值？

Question

有一個查詢：

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; 

有兩個表tmp和tmp1工會合並，但增加了一倍場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，我只需要最後一個記錄（消息或明信片 - 不沒有問題）和最後一個記錄的ID（有問題 - 我可以在郵件和明信片單獨獲取最大（id），但不能在合併表中完成）：

privmsgs_id contact privmsgs_date contentType 
21556868 7070 1315215266  postcard 
21226460 7754 1312025735  message 
21489812 15588 1315208838  message 

原因，爲什麼我不通過簡化查詢來做到這一點，因爲我需要特定數量的結果，因此，我只能通過一個查詢來完成此操作。

Answer 1

您必須將結果分組爲contact並選擇最大值privmsgd_id，然後從中選擇僅來自privmsgd_id的信息。

查看GROUP BY瞭解更多信息。