文章出處

在一個SQL Server表中一行的多個列找出最大值

有時候我們需要從多個相同的列里（這些列的數據類型相同）找出最大的那個值，并顯示

這里給出一個例子

IF (OBJECT_ID('tempdb..##TestTable') IS NOT NULL)
    DROP TABLE ##TestTable

CREATE TABLE ##TestTable
(
    ID INT IDENTITY(1,1) PRIMARY KEY,
    Name NVARCHAR(40),
    UpdateByApp1Date DATETIME,
    UpdateByApp2Date DATETIME,
    UpdateByApp3Date DATETIME

)

INSERT INTO ##TestTable(Name, UpdateByApp1Date, UpdateByApp2Date, UpdateByApp3Date )
VALUES('ABC', '2015-08-05','2015-08-04', '2015-08-06'),
      ('NewCopmany', '2014-07-05','2012-12-09', '2015-08-14'),
      ('MyCompany', '2015-03-05','2015-01-14', '2015-07-26')
      
SELECT * FROM ##TestTable

結果如下所示

有三種方法可以實現

方法一

SELECT  ID ,
        Name ,
        ( SELECT    MAX(LastUpdateDate)
          FROM      ( VALUES ( UpdateByApp1Date), ( UpdateByApp2Date),
                    ( UpdateByApp3Date) ) AS UpdateDate ( LastUpdateDate )
          ) AS LastUpdateDate
FROM    ##TestTable

方法二

SELECT  ID ,
        [Name] ,
        MAX(UpdateDate) AS LastUpdateDate
FROM    ##TestTable UNPIVOT ( UpdateDate FOR DateVal IN ( UpdateByApp1Date,
                                                          UpdateByApp2Date,
                                                          UpdateByApp3Date ) ) AS u
GROUP BY ID ,
        Name

方法三

SELECT  ID ,
        name ,
        ( SELECT    MAX(UpdateDate) AS LastUpdateDate
          FROM      ( SELECT    tt.UpdateByApp1Date AS UpdateDate
                      UNION
                      SELECT    tt.UpdateByApp2Date
                      UNION
                      SELECT    tt.UpdateByApp3Date
                    ) ud
        ) LastUpdateDate
FROM    ##TestTable tt

第一種方法使用values子句，將每行數據構造為只有一個字段的表，以后求最大值，非常巧妙

第二種方法使用行轉列經常用的UNPIVOT 關鍵字進行轉換再顯示

第三種方法跟第一種方法差不多，但是使用union將三個UpdateByAppDate字段合并為只有一個字段的結果集然后求最大值

第一種方法的執行計劃

第二種方法的執行計劃

第三種方法的執行計劃

總的來說，第一種方法的執行計劃是最好的

注意，這里不涉及分組

IF (OBJECT_ID('tempdb..##TestTable') IS NOT NULL)
    DROP TABLE ##TestTable

CREATE TABLE ##TestTable
    (
      ID INT IDENTITY(1, 1)
             PRIMARY KEY ,
      Name NVARCHAR(40) ,
      UpdateByApp1Date DATETIME ,
      UpdateByApp2Date DATETIME ,
      UpdateByApp3Date DATETIME
    )

INSERT  INTO ##TestTable
        ( Name, UpdateByApp1Date, UpdateByApp2Date, UpdateByApp3Date )
VALUES  ( 'ABC', '2015-08-05', '2015-08-04', '2015-08-06' ),
        ( 'ABC', '2015-07-05', '2015-06-04', '2015-09-06' ),
        ( 'NewCopmany', '2014-07-05', '2012-12-09', '2015-08-14' ),
        ( 'MyCompany', '2015-03-05', '2015-01-14', '2015-07-26' )
      
SELECT  *
FROM    ##TestTable

SELECT  ID ,
        Name ,
        ( SELECT    MAX(LastUpdateDate)
          FROM      ( VALUES ( UpdateByApp1Date), ( UpdateByApp2Date),
                    ( UpdateByApp3Date) ) AS UpdateDate ( LastUpdateDate )
          ) AS LastUpdateDate
FROM    ##TestTable