Sql 2008R2 Max（）

Question

我有一个函数，我写了以下几行。

DECLARE @MaxLetterID BigInt;
DECLARE @MaxObserverLetterID BigInt;
DECLARE @MaxBillLetterID bigInt;

SET @MaxLetterID = (SELECT     MAX(LetterNO) AS Expr1
                    FROM         Lab
                    WHERE     (LetterNO BETWEEN @Min AND @Max));

SET @MaxObserverLetterID = (SELECT     MAX(LetterNO) AS Expr1
                    FROM         ObserverCoReq
                    WHERE     (LetterNO BETWEEN @Min AND @Max));

SET @MaxBillLetterID =(SELECT     MAX(LetterNO) AS Expr1
                    FROM         BillLetters
                    WHERE     (LetterNO BETWEEN @Min AND @Max));

SET @FinalID=MAX(@MaxLetterID,@MaxObserverLetterID,@MaxBillLetterID)

我是对的最后一行吗？我可以像这样使用Max功能吗？

Answer 1

不，你不能这样做。您需要一个SQL Server没有的greatest功能。但是这个

select @FinalID = max(c)  
from 
(
     select @MaxLetterID as c
     union  
     select @MaxObserverLetterID as c
     union
     select @MaxBillLetterID as c
) x

SQLFiddle demo