SQL：为每个唯一键选择最大值？

Question

抱歉，我不确定该如何表述，而且我对SQL真的不太满意。 数据库引擎i SQL Server Compact。 我目前有这个查询：

SELECT *
FROM Samples
WHERE FunctionId NOT IN
(SELECT CalleeId FROM Callers)
ORDER BY ThreadId, HitCount DESC

这给了我：

ThreadId   Function  HitCount
       1        164      6945
       1       3817         1
       4       1328      7053

现在，我只希望结果具有Thread每个唯一值的最大命中计数。 换句话说，应该删除第二行。 我不确定如何实现这一目标。

[EDIT]如果有帮助，这是同一查询的替代形式：

SELECT *
FROM Samples s1
LEFT OUTER JOIN Callers c1
    ON s1.ThreadId = c1.ThreadId AND s1.FunctionId = c1.CalleeId
WHERE c1.ThreadId IS NULL
ORDER BY ThreadId

[编辑]我最终进行了模式更改，以避免这样做，因为建议的查询看起来相当昂贵。 感谢您的所有帮助。

Answer 1

这是我的处理方式：

SELECT s1.*
FROM Samples s1
LEFT JOIN Samples s2 
  ON (s1.Thread = s2.Thread and s1.HitCount < s2.HitCount)
WHERE s1.FunctionId NOT IN (SELECT CalleeId FROM Callers) 
  AND s2.Thread IS NULL
ORDER BY s1.ThreadId, s1.HitCount DESC

换句话说，没有其他行s2匹配相同Thread并具有更大HitCount的行s1 。

Answer 2

SQL Server Compact是否支持窗口函数？

备选方案1-将包括所有绑定的行。 如果给定线程的所有行的HitCount都为null，则将不包含行：

SELECT Thread, Function, HitCount
FROM (SELECT Thread, Function, HitCount,
        MAX(HitCount) over (PARTITION BY Thread) as MaxHitCount
    FROM Samples
    WHERE FunctionId NOT IN
        (SELECT CalleeId FROM Callers)) t 
WHERE HitCount = MaxHitCount 
ORDER BY ThreadId, HitCount DESC

备选方案2-将包括所有绑定的行。 如果给定线程的HitCount不为空，则没有行，则将返回该线程的所有行：

SELECT Thread, Function, HitCount
FROM (SELECT Thread, Function, HitCount,
        RANK() over (PARTITION BY Thread ORDER BY HitCount DESC) as R
    FROM Samples
    WHERE FunctionId NOT IN
        (SELECT CalleeId FROM Callers)) t
WHERE R = 1
ORDER BY ThreadId, HitCount DESC

备选方案3-在出现平局的情况下将不确定地选择一行，并丢弃其他行。 如果给定线程的所有行的空HitCount将包括一行

SELECT Thread, Function, HitCount
FROM (SELECT Thread, Function, HitCount,
        ROW_NUMBER() over (PARTITION BY Thread ORDER BY HitCount DESC) as R
    FROM Samples
    WHERE FunctionId NOT IN
        (SELECT CalleeId FROM Callers)) t
WHERE R = 1
ORDER BY ThreadId, HitCount DESC

替代4和5-如果窗口函数不可用，则使用较旧的构造，并表示比使用联接更干净。 优先考虑基准测试。 两者都返回参与平局的所有行。 当非空值不可用于HitCount时，备选方案4将HitCount为null。 备选方案5将不会返回HitCount为null的行。

SELECT *
FROM Samples s1
WHERE FunctionId NOT IN
    (SELECT CalleeId FROM Callers)
AND NOT EXISTS
    (SELECT *
    FROM Samples s2
    WHERE s1.FunctionId = s2.FunctionId
    AND s1.HitCount < s2.HitCount)
ORDER BY ThreadId, HitCount DESC

SELECT *
FROM Samples s1
WHERE FunctionId NOT IN
    (SELECT CalleeId FROM Callers)
AND HitCount = 
    (SELECT MAX(HitCount)
    FROM Samples s2
    WHERE s1.FunctionId = s2.FunctionId)
ORDER BY ThreadId, HitCount DESC

Answer 3

适用于SQL Server 2005+：

WITH maxHits AS(
  SELECT s.threadid,
         MAX(s.hitcount) 'maxhits'
    FROM SAMPLES s
    JOIN CALLERS c ON c.threadid = s.threadid AND c.calleeid != s.functionid
GROUP BY s.threadid
)
SELECT t.*
  FROM SAMPLES t
  JOIN CALLERS c ON c.threadid = t.threadid AND c.calleeid != t.functionid
  JOIN maxHits mh ON mh.threadid = t.threadid AND mh.maxhits = t.hitcount

在任何数据库上工作：

SELECT t.*
  FROM SAMPLES t
  JOIN CALLERS c ON c.threadid = t.threadid AND c.calleeid != t.functionid
  JOIN (SELECT s.threadid,
               MAX(s.hitcount) 'maxhits'
          FROM SAMPLES s
          JOIN CALLERS c ON c.threadid = s.threadid AND c.calleeid != s.functionid
      GROUP BY s.threadid) mh ON mh.threadid = t.threadid AND mh.maxhits = t.hitcount