请尝试使用UNPIVOT:

SELECT MAX(MaxDt) MaxDt
   FROM tbl 
UNPIVOT
   (MaxDt FOR E IN 
      (Date1, Date2, Date3)
)AS unpvt;

上表为员工工资表，列为salary1、salary2、salary3、salary4。下面的查询将返回四列中的最大值

select  
 (select Max(salval) from( values (max(salary1)),(max(salary2)),(max(salary3)),(max(Salary4)))alias(salval)) as largest_val
 from EmployeeSalary

运行上述查询将输出largest_val(10001)

上述查询逻辑如下:

select Max(salvalue) from(values (10001),(5098),(6070),(7500))alias(salvalue)

输出将是10001

SELECT 
    CASE 
        WHEN Date1 >= Date2 AND Date1 >= Date3 THEN Date1 
        WHEN Date2 >= Date3 THEN Date2 
        ELSE Date3
    END AS MostRecentDate 

这样写起来稍微容易一些，因为case语句是按顺序求值的，因此跳过了求值步骤。

For T-SQL (MSSQL 2008+)

SELECT
  (SELECT
     MAX(MyMaxName) 
   FROM ( VALUES 
            (MAX(Field1)), 
            (MAX(Field2)) 
        ) MyAlias(MyMaxName)
  ) 
FROM MyTable1

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

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