試圖簡化MySQL查詢

Question

我真的很感謝任何幫助，試圖簡化MySQL查詢。查詢的目的是從消息表（users_messages）中檢索具有以下列的消息：message_id，from_id，to_id，message_content，date_sent。

from_id和to_id需要加入具有以下列的用戶表（用戶）：user_id，user_username。

另外我應該提到的是，有一個阻塞的用戶表（users_blocked）可以過濾掉這個表中的user_id功能。

所有這些工作正常，消息與最新的第一個是我想要的。我唯一的問題是它沒有拉動相應的'message_content'。即它拉動最近的日期，但不是最近的消息。

也許我需要一種不同的方法（例如子查詢），但我無法繞過它。

下面是該查詢：

select m.message_content, 
    if(from_id < to_id, concat(from_id,to_id), concat(to_id,from_id)) as ft, 
    if (from_id = $my_id, to_id, from_id) as other_id, 
    max(date_sent) as most_recent 
from users_messages m 
    left join users_blocked ub1 on (from_id = ub1.blocked_id and ub1.user_id = $my_id) 
    left join users_blocked ub2 on (to_id = ub2.blocked_id and ub2.user_id = $my_id) 
where 
    (from_id = $my_id or to_id = $my_id) 
    and ub1.blocked_id is null 
    and ub2.blocked_id is null 
group by 
    ft 
order by 
    most_recent desc 

對不起，這裏的表結構：

用戶

user_id user_username 
1   Simon 
2   Amber 
3   Tom

users_messages

 
message_id from_id to_id date_sent    message_content 
1    1   2  2012-07-04 11:52:12 Hello 
2    1   2  2012-07-04 12:32:24 Another message 
3    1   2  2012-07-04 14:00:00 Hello again 

users_blocked

 
user_id blocked_id 
1   3 

Answer 1

嘗試：

select m.message_content, 
     x.ft, 
     x.other_id, 
     x.most_recent 
from (select if(from_id < to_id, concat(from_id,to_id), concat(to_id,from_id)) as ft, 
      if(from_id = $my_id, to_id, from_id) as other_id, 
      max(date_sent) as most_recent 
     from users_messages um 
     left join users_blocked ub1 
     on (um.from_id = ub1.blocked_id and ub1.user_id = $my_id) 
     left join users_blocked ub2 
     on (um.to_id = ub2.blocked_id and ub2.user_id = $my_id) 
     where ub1.blocked_id is null and ub2.blocked_id is null and 
      (um.from_id = $my_id or um.to_id = $my_id) 
     group by ft) x 
join users_messages m 
    on m.date_sent = x.most_recent and 
     m.from_id in ($my_id, x.other_id) and 
     m.to_id in ($my_id, x.other_id) 
order by 
    x.most_recent desc 

SQLFiddle here。

Answer 2

這裏我假定SimonKing想，從users_messages表，其包括以下條件的消息內容，

1. 用戶不應阻止任一方向，
2. 用戶之間傳送的最新的郵件

這樣，我修改了Mark Ba​​nnister的查詢如下，

SELECT temp.* FROM (
SELECT um.*, concat(um.from_id,to_id) as direction FROM userMessages um 
LEFT JOIN userBlocked ub1 ON um.from_id = ub1.user_id AND um.to_id = ub1.blocked_id 
LEFT JOIN userBlocked ub2 ON um.to_id = ub2.user_id AND um.from_id = ub2.blocked_id 
WHERE ub1.user_id is null AND ub1.blocked_id is null AND ub2.user_id is null AND ub2.blocked_id is null 
ORDER BY um.date_sent DESC 
) temp 
GROUP BY direction 

SQL小提琴是http://sqlfiddle.com/#!2/bdc77/1/0

Answer 3

按照我的理解，這個請求的主要問題是，結果只包含第一日期，而不是消息。爲了解決這個問題，你可以這樣做：

1. 化妝準備的數據集，這將有最新的日期：

   選擇to_id，from_id，MAX（DATE_SENT）作爲most_recent 從users_messages中號 左連接users_blocked UB在ub.user_id = $ my_id 和ub。blocked_id在（to_id，from_id） 其中 （from_id = $添加my_id或to_id = $添加my_id） 和ub.blocked_id爲空 組由 to_id，通過 most_recent降序from_id 順序

我看到你用兩個collumns to_id，from_id來摸索數據。此子查詢不是計算類的東西最好的地方：

if(from_id < to_id, concat(from_id,to_id), concat(to_id,from_id)) as ft 

1. 然後，只需選擇users_messages其他需要的數據，這符合我們to_id，from_id和recent_date從準備表：

   選擇微米。 *從

   (
   select to_id, from_id, max(date_sent) as most_recent 
   from users_messages m 
   left join users_blocked ub on ub.user_id = 1 
   and ub.blocked_id in (to_id, from_id) 
   where 
   (from_id = 1 or to_id = 1) 
   and ub.blocked_id is null 
   group by 
   to_id, from_id 
   order by 
   most_recent desc 
   ) as prepared_messages 
   left join users_messages um on um.from_id = prepared_messages.from_id 
   and um.to_id = prepared_messages.to_id 
   and um.date_sent = prepared_messages.most_recent