确保从MySQL查询返回正确的行数

Question

我有一个（游戏）排行榜表，其中包含由另一个查询更新的排名。它以ID为中心 - 当我查询表时，我最好想要取回N条记录。所以，我已经做了UNION在记录都与限制N/2，像这样：

(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 

尽管这并不完全工作的时候在领先榜首位，它只会返回较低的N/2。有没有办法确保它总能返回N条记录？我确实认为你可以有目的地获得比需要更多的记录，然后修剪你不需要的记录。虽然我不知道如何用查询来做到这一点！

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Answer 1

你可以用一个巧妙的运用order by和limit做到这一点：

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; 

你可能再想这些早在排名顺序为：

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

请尝试以下操作：

(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了解更多信息。