基于http://www.experts-exchange.com/Microsoft/Development/MS-SQL-Server/Q_24204894.html的ScottPletcher的解决方案 我已经创建了一组函数(例如GetMaxOfDates3, GetMaxOfDates13)，以使用UNION ALL找到最多13个日期值。 参见T-SQL函数从同一行获取最大值 然而，在编写这些函数时，我还没有考虑过UNPIVOT解决方案

CREATE FUNCTION GetMaxOfDates13 (
@value01 DateTime = NULL,  
@value02 DateTime = NULL,
@value03 DateTime = NULL,
@value04 DateTime = NULL,
@value05 DateTime = NULL,
@value06 DateTime = NULL,
@value07 DateTime = NULL,
@value08 DateTime = NULL,
@value09 DateTime = NULL,
@value10 DateTime = NULL,
@value11 DateTime = NULL,
@value12 DateTime = NULL,
@value13 DateTime = NULL
)
RETURNS DateTime
AS
BEGIN
RETURN (
SELECT TOP 1 value
FROM (
SELECT @value01 AS value UNION ALL
SELECT @value02 UNION ALL
SELECT @value03 UNION ALL
SELECT @value04 UNION ALL
SELECT @value05 UNION ALL
SELECT @value06 UNION ALL
SELECT @value07 UNION ALL
SELECT @value08 UNION ALL
SELECT @value09 UNION ALL
SELECT @value10 UNION ALL
SELECT @value11 UNION ALL
SELECT @value12 UNION ALL
SELECT @value13
) AS [values]
ORDER BY value DESC    
)
END –FUNCTION
GO
CREATE FUNCTION GetMaxOfDates3 (
@value01 DateTime = NULL,  
@value02 DateTime = NULL,
@value03 DateTime = NULL
)
RETURNS DateTime
AS
BEGIN
RETURN dbo.GetMaxOfDates13(@value01,@value02,@value03,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL)
END –FUNCTION

从SQL Server 2012我们可以使用IIF。

 DECLARE @Date1 DATE='2014-07-03';
 DECLARE @Date2 DATE='2014-07-04';
 DECLARE @Date3 DATE='2014-07-05';

 SELECT IIF(@Date1>@Date2,
        IIF(@Date1>@Date3,@Date1,@Date3),
        IIF(@Date2>@Date3,@Date2,@Date3)) AS MostRecentDate

您可以创建一个函数，其中传递日期，然后将该函数添加到select语句，如下所示。 select Number, dbo.fxMost_Recent_Date(Date1,Date2,Date3)，成本

create FUNCTION  fxMost_Recent_Date 

（ @Date1 smalldatetime, @Date2 smalldatetime, @Date3 smalldatetime ） 返回smalldatetime 作为 开始 DECLARE @Result smalldatetime

declare @MostRecent smalldatetime

set @MostRecent='1/1/1900'

if @Date1>@MostRecent begin set @MostRecent=@Date1 end
if @Date2>@MostRecent begin set @MostRecent=@Date2 end
if @Date3>@MostRecent begin set @MostRecent=@Date3 end
RETURN @MostRecent

END

我的解决方案也可以处理空值比较。它可以被简化为一个单一的查询，但为了解释，我使用CTE。这个想法是在第一步中减少从3个数字到2个数字的比较，然后在第二步中从2个数字到1个数字的比较。

with x1 as
(
  select 1 as N1, null as N2, 3 as N3
  union
  select 1 as N1, null as N2, null as N3
  union
  select null as N1, null as N2, null as N3
)
,x2 as
(
select 
N1,N2,N3,
IIF(Isnull(N1,0)>=Isnull(N2,0),N1,N2) as max1,
IIF(Isnull(N2,0)>=Isnull(N3,0),N2,N3) as max2
from x1
)
,x3 as
(
 select N1,N2,N3,max1,max2,
 IIF(IsNull(max1,0)>=IsNull(max2,0),max1,max2) as MaxNo
 from x2
)
select * from x3

输出:

我不知道它是否在SQL上，等等…在M$ACCESS帮助中，有一个称为MAXA(Value1;Value2;…)的函数，它应该这样做。

希望能帮助到别人。

p.d.:值可以是列或计算值，等等。

还有3种方法，其中UNPIVOT(1)是目前为止最快的，其次是模拟UNPIVOT(3)，它比(1)慢得多，但仍然比(2)快得多

CREATE TABLE dates
    (
      number INT PRIMARY KEY ,
      date1 DATETIME ,
      date2 DATETIME ,
      date3 DATETIME ,
      cost INT
    )

INSERT  INTO dates
VALUES  ( 1, '1/1/2008', '2/4/2008', '3/1/2008', 10 )
INSERT  INTO dates
VALUES  ( 2, '1/2/2008', '2/3/2008', '3/3/2008', 20 )
INSERT  INTO dates
VALUES  ( 3, '1/3/2008', '2/2/2008', '3/2/2008', 30 )
INSERT  INTO dates
VALUES  ( 4, '1/4/2008', '2/1/2008', '3/4/2008', 40 )
GO

解决方案1 (UNPIVOT)

SELECT  number ,
        MAX(dDate) maxDate ,
        cost
FROM    dates UNPIVOT ( dDate FOR nDate IN ( Date1, Date2,
                                            Date3 ) ) as u
GROUP BY number ,
        cost 
GO

解决方案2(每行子查询)

SELECT  number ,
        ( SELECT    MAX(dDate) maxDate
          FROM      ( SELECT    d.date1 AS dDate
                      UNION
                      SELECT    d.date2
                      UNION
                      SELECT    d.date3
                    ) a
        ) MaxDate ,
        Cost
FROM    dates d
GO

方案3(模拟UNPIVOT)

;WITH    maxD
          AS ( SELECT   number ,
                        MAX(CASE rn
                              WHEN 1 THEN Date1
                              WHEN 2 THEN date2
                              ELSE date3
                            END) AS maxDate
               FROM     dates a
                        CROSS JOIN ( SELECT 1 AS rn
                                     UNION
                                     SELECT 2
                                     UNION
                                     SELECT 3
                                   ) b
               GROUP BY Number
             )
    SELECT  dates.number ,
            maxD.maxDate ,
            dates.cost
    FROM    dates
            INNER JOIN MaxD ON dates.number = maxD.number
GO

DROP TABLE dates
GO