SQL：为每个唯一键选择最大值？

Question

抱歉，我不知道该如何表达，而且我真的不太擅长 SQL。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，我只想要 Thread 的每个唯一值的最大命中数的结果。换句话说，应该删除第二行。我不知道如何解决这个问题。

[编辑] 如果有帮助，这是同一查询的另一种形式：

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] 我最终进行了架构更改以避免这样做，因为建议的查询看起来相当昂贵。感谢大家的帮助。

Answer 1

SQL Server compact 是否支持窗口函数？

备选方案 1--将包括所有并列的行。如果给定 Thread 的唯一行的 HitCount 都为 null，则不会包含一行：

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--将包括所有并列的行。如果给定线程没有具有非空 HitCount 的行，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--如果出现平局，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 HitCount 都为空，则将包含一行

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

Alternative 4 & 5--如果窗口函数不可用，则使用较旧的构造，并说明比使用连接更简洁的含义。如果速度是优先级，则进行基准测试。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 为空。备选方案 5 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 2

我会这样做：

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并且具有更大HitCount的行s1。

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