SQL：如何在分組的查詢中查找頂部的字符串列值

Question

當進行分組時，有一種簡單的方法可以從列中獲取第一個值或最高值。 在下面的示例中，如果按[日期]降序排序，我希望[文本]中的最后一個值。

SELECT 
   [Id],
   MAX([Date]),
   TOP 1 [Text],  <-- How do I do do this
   COUNT(*)
FROM
   [ExampleTable]
GROUP BY [Id]
ORDER BY [Date] DESC

提前致謝。

Answer 1

如果您想要與[Id]和[MaxDate]相對應的文本，則可以執行以下操作：

SELECT T2.*, T1.Text
FROM [ExampleTable] T1
     JOIN (SELECT 
               [Id],
               MAX([Date]) MaxDate,
               COUNT(*) Cnt
             FROM
               [ExampleTable]
         GROUP BY [Id]) T2
     ON T1.Id = T2.Id AND T1.Date = T2.MaxDate
ORDER BY [Date] DESC

如果[Id]和[Date]構成主鍵，則每個（Id，Date）都會有一個單行，否則很多。 如果您真的想要一個，則可以放置其他約束，也可以使用MAX或MIN ::

SELECT T2.*, MAX(T1.Text) Text -- or MIN(T1.Text)
FROM [ExampleTable] T1
     JOIN (SELECT 
               [Id],
               MAX([Date]) MaxDate,
               COUNT(*) Cnt
             FROM
               [ExampleTable]
         GROUP BY [Id]) T2
     ON T1.Id = T2.Id AND T1.Date = T2.MaxDate
GROUP BY T2.*
ORDER BY [Date] DESC

如果只需要一個Text值，則可以使用MAX或MIN：

SELECT 
   [Id],
   MAX([Date]),
   MAX([Text]),
   COUNT(*)
FROM
   [ExampleTable]
GROUP BY [Id]
ORDER BY [Date] DESC

Answer 2

如果我正確理解您的意見，則任何給定ID可能有多個日期，給定日期可能有多個文本。

如果使用Sql Server 2005

我有這個工作

DECLARE @Table TABLE(
        ID INT,
        DDATE DATETIME,
        VAL VARCHAR(MAX)
)

INSERT INTO @Table (ID,DDATE,VAL) SELECT 1, '01 Jan 2009', '1'
INSERT INTO @Table (ID,DDATE,VAL) SELECT 1, '01 Feb 2009', '2'
INSERT INTO @Table (ID,DDATE,VAL) SELECT 1, '01 Feb 2009', '3'
INSERT INTO @Table (ID,DDATE,VAL) SELECT 2, '01 Jan 2009', '4'


SELECT  ID,
        DDATE,
        VAL,
        ROWNUMBER
FROM    (
            SELECT  t.ID,
                    t.DDATE,
                    t.VAL,
                    ROW_NUMBER() OVER (PARTITION BY t.ID ORDER BY DDATE) ROWNUMBER
            FROM    @Table t INNER JOIN
                    (
                        SELECT  ID,
                                MAX(DDATE) AS MAXDATE
                        FROM    @Table
                        GROUP BY ID
                    ) md    ON  t.ID = md.ID
                            AND t.DDATE = md.MAXDATE
        ) RowNumber
WHERE   RowNumber.ROWNUMBER = 1