有时候我们需要从多个相同的列里（这些列的数据类型相同）找出最大的那个值，并显示
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这里给出一个例子
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
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有三种方法可以实现
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方法一
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SELECT ID ,
Name ,
( SELECT MAX(LastUpdateDate)
FROM ( VALUES ( UpdateByApp1Date), ( UpdateByApp2Date),
( UpdateByApp3Date) ) AS UpdateDate ( LastUpdateDate )
) AS LastUpdateDate
FROM ##TestTable
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方法二
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SELECT ID ,
[Name] ,
MAX(UpdateDate) AS LastUpdateDate
FROM ##TestTable UNPIVOT ( UpdateDate FOR Dateva l 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
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?
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第一种方法使用values子句，将每行数据构造为只有一个字段的表，以后求最大值，非常巧妙
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第二种方法使用行转列经常用的UNPIVOT 关键字进行转换再显示
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第三种方法跟第一种方法差不多，但是使用union将三个UpdateByAppDate字段合并为只有一个字段的结果集然后求最大值
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第一种方法的执行计划
第二种方法的执行计划
第三种方法的执行计划
总的来说，第一种方法的执行计划是最好的
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注意，这里不涉及分组
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
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?
name列相同的话，是无法得出name分组之后的最大值，这里要注意一下
