確保從MySQL查詢返回正確的行數

Question

我有一個（遊戲）排行榜表，其中包含由另一個查詢更新的排名。它以ID爲中心 - 當我查詢表時，我最好想要取回N條記錄。所以，我已經做了UNION在記錄都與限制N/2，像這樣：

(SELECT * FROM test1 t WHERE t.rank > (SELECT rank FROM test1 t2 WHERE id=ID)+0 ORDER BY rank LIMIT 0, N/2) 
UNION ALL 
(SELECT * FROM test1 t WHERE t.rank <= (SELECT rank FROM test1 t2 WHERE id=ID)+0 ORDER BY rank desc LIMIT 0, N/2) ORDER BY rank 

儘管這並不完全工作的時候在領先榜首位，它只會返回較低的N/2。有沒有辦法確保它總能返回N條記錄？我確實認爲你可以有目的地獲得比需要更多的記錄，然後修剪你不需要的記錄。雖然我不知道如何用查詢來做到這一點！

任何幫助表示讚賞:)

Answer 1

你可以用一個巧妙的運用order by和limit做到這一點：

SELECT t.* 
FROM test1 t cross join 
    (SELECT rank FROM test1 t2 WHERE id = @ID) as theone 
ORDER BY ABS(theone.rank - t.rank) 
LIMIT N; 

你可能再想這些早在排名順序爲：

SELECT t.* 
FROM (SELECT t.* 
     FROM test1 t cross join 
      (SELECT rank FROM test1 t2 WHERE id = @ID) as theone 
     ORDER BY ABS(theone.rank - t.rank) 
     LIMIT N 
    ) t 
ORDER BY t.rank; 

Answer 2

請嘗試以下操作：

(SELECT * FROM test1 t WHERE t.rank > (SELECT rank FROM test1 t2 WHERE id=ID)+0 ORDER BY rank LIMIT 0,N) 
UNION ALL 
(SELECT * FROM test1 t WHERE t.rank <= (SELECT rank FROM test1 t2 WHERE id=ID)+0 ORDER BY rank desc LIMIT 0,N) 
ORDER BY rank 
LIMIT 0,N 

查看manual瞭解更多信息。