MAX 日期子查询 SQL Server答案

【问题标题】：MAX Date Subquery SQL ServerMAX 日期子查询 SQL Server
【发布时间】：2018-02-14 14:50:42
【问题描述】：

我正在尝试在子查询中加入最大日期，但出现错误。我正在使用 SQL 服务器

SELECT Headcount.[Snapshot Date]
, Headcount.[ID]
, Assessment.[Assessment Date]
, Assessment.[Code]

FROM Headcount INNER JOIN Assessment ON Assessment.[ID] = Headcount.[ID]
WHERE Assessment.[Assessment Date] = 
                 (SELECT MAX(CAST(Assessment.[Assessment Date] as datetime)) 
                  FROM Assessmnet AS Z 
                  WHERE Z.[ID] = Assessment.[ID] 
                       AND Assessment.[Assessment Date] <= Headcount.[Snapshot Date]
                 )

但我得到一个错误：

聚合可能不会出现在 WHERE 子句中，除非它出现在 HAVING 子句或选择列表中包含的子查询中，并且被聚合的列是外部引用。

【问题讨论】：

标签： sql sql-server group-by

【解决方案1】：

您忘记了在子查询中指定的别名 Z。如果您在子查询中使用Assessment，那么他会考虑来自外部查询的Assessment，因为Assessment 在您的子查询中具有名称Z。

SELECT Headcount.[Snapshot Date]
, Headcount.[ID]
, Assessment.[Assessment Date]
, Assessment.[Code]

FROM Headcount INNER JOIN Assessment ON Assessment.[ID] = Headcount.[ID]
WHERE Assessment.[Assessment Date] = 
                 (SELECT MAX(CAST(Z.[Assessment Date] as datetime)) 
                  FROM Assessment AS Z 
                  WHERE Z.[ID] = Assessment.[ID] 
                       AND Z.[Assessment Date] <= Headcount.[Snapshot Date]
                 )

【讨论】：

【解决方案2】：

问题出在where 条件的子查询中，请尝试以下操作：

SELECT
    Headcount.[Snapshot Date],
    Headcount.[ID],
    Assessment.[Assessment Date],
    Assessment.[Code]
FROM Headcount
INNER JOIN Assessment ON Assessment.[ID] = Headcount.[ID]
WHERE Assessment.[Assessment Date] =
(
    SELECT
        MAX(CAST(Assessment.[Assessment Date] AS DATETIME))
    FROM Assessmnet AS Z
    GROUP BY
        Z.ID
    HAVING Z.[ID] = Assessment.[ID]
           AND Assessment.[Assessment Date] <= Headcount.[Snapshot Date]
);

【讨论】：

【解决方案3】：

你应该写MAX(CAST(Z.[Assessment Date] as datetime))而不是MAX(CAST(Assessment.[Assessment Date] as datetime))。否则，您将在错误的表中进行聚合。

以下查询可能有效（未经测试！）：

SELECT Headcount.[Snapshot Date]
, Headcount.[ID]
, Assessment.[Assessment Date]
, Assessment.[Code]
FROM Headcount
INNER JOIN Assessment ON Assessment.[ID] = Headcount.[ID]
WHERE Assessment.[Assessment Date] =
                 (SELECT MAX(CAST(Z.[Assessment Date] as datetime))
                  FROM Assessment AS Z
                  WHERE Z.[ID] = Assessment.[ID] 
                    AND Z.[Assessment Date] <= Headcount.[Snapshot Date])

您也可以重写它以使用ROW_NUMBER 为每个评估选择最近的一行：

SELECT [Snapshot Date], [ID], [Assessment Date], [Code]
FROM (SELECT [Snapshot Date], [ID], [Assessment Date], [Code],
        ROW_NUMBER() OVER (PARTITION BY [ID] ORDER BY [Assessment Date] DESC) rn
      FROM Headcount JOIN Assessment USING ([ID])
      WHERE [Assessment Date] <= [Snapshot Date]) t
WHERE rn = 1

【讨论】：