获取外键的最后一个实例

Question

我想查询一个表并返回与每个外键实例关联的最新日期。例如：

换句话说，我希望我开始使用ExpenditureDate中具有最高价值的每个项目的支出，使用以下查询来获取最后一次支出，但这是错误的，因为添加的最后一个不是永远是最新的日期：

WITH LatestCumulativeExpenditure AS
(
    SELECT DISTINCT
        MAX(p.Id) AS Id
    FROM
        tbl_ProjectExpenditure p
    GROUP BY
        p.ProjectID
)
SELECT
    pe.Id,
    pe.ProjectID,
    pe.CumulativeMonthlyExpenditure
FROM
    tbl_ProjectExpenditure pe
    INNER JOIN LatestCumulativeExpenditure lce
        ON pe.Id = lce.Id
    INNER JOIN tbl_Project p
        ON pe.ProjectID = p.Id

Answer 1

而不是查询MAX(p.Id)，而是查询MAX(ExpenditureDate)。您还需要其他列，例如ProjectID，并加入两列。

WITH LatestCumulativeExpenditure AS
(
    SELECT DISTINCT
        -- Get both the ProjectID and the max ExpenditureDate
        ProjectID,
        MAX(ExpenditureDate) AS maxdate
    FROM
        tbl_ProjectExpenditure
    GROUP BY
        ProjectID
)
SELECT
    pe.Id,
    pe.ProjectID,
    pe.CumulativeMonthlyExpenditure
FROM
    tbl_ProjectExpenditure pe
    INNER JOIN tbl_Project p
        ON pe.ProjectID = p.Id
    -- Join *both* columns against the primary tbl_ProjectExpenditure
    -- to match the row with that expenditure date per project Id
    INNER JOIN LatestCumulativeExpenditure lce
        ON pe.ProjectID = lce.ProjectID
        AND pe.ExpenditureDate = lce.maxdate

（编辑 - 抱歉，顶部说明中的列名错误）

Answer 2

以下内容如何：

SELECT p.Id, p.ProjectID,
(
  SELECT CumulativeMonthlyExpenditure
  FROM tbl_ProjectExpenditure pe
  WHERE pe.ProjectID = p.Id
  ORDER BY ExpenditureDate DESC
  LIMIT 1
) AS MAX_CumulativeMonthlyExpenditure
FROM tbl_Project p