Question

我有以下查询，您可以看到多个Count(CompetitorID)来电。这是性能问题，还是SQL Server 2008'缓存'Count？如果这是性能问题，是否可以存储Count以防止多次查找？

SELECT EventID,Count(CompetitorID) AS NumberRunners,
    CASE WHEN Count(CompetitorID)<5 THEN 1
         WHEN Count(CompetitorID)>=5 AND Count(CompetitorID)<=7 THEN 2
         ELSE 3 END AS NumberPlacings
FROM Comps
GROUP BY EventID Order By EventID;

Answer 1

获取一次值并随后尽可能使用它总是更好的做法。在您的情况下，您始终可以使用内部查询仅计算一次计数并从其值计算其他（派生）列，如下所示：

SELECT  EventID, NumberRunners,
        CASE WHEN NumberRunners <5 THEN 1
             WHEN NumberRunners >=5 AND NumberRunners <=7 THEN 2
             ELSE 3 
        END AS NumberPlacings
  FROM  (
            SELECT  EventID,
                    NumberRunners = Count(CompetitorID)
              FROM  Comps
            GROUP BY EventID
        ) t
Order By EventID;

Answer 2

最简单的是：

SELECT EventID，Count（distinct CompetitorID）AS NumberRunners，计数（不同的竞争者ID）＆lt; 5那么1 当计数（不同的竞争者ID）＆gt; = 5和计数（不同的竞争者ID）＆lt; = 7那么2 ELSE 3 END作为NumberPlacings FROM Comps GROUP BY EventID按EventID排序;

在SELECT查询中防止重复的COUNT

2 个答案: