请教一个sql问题，暂时放在首页

Question

declare @CustomerID uniqueidentifier <P><BR>set @CustomerID = '129F1B78-5373-4196-BE92-253762896ADB' </P> <P>&nbsp;</P> <P><BR>select a.* from opvoucher a <BR>WHERE&nbsp;&nbsp;&nbsp; (a.customercode = @CustomerID or @CustomerID is null ) </P> <P>&nbsp;</P> <P><BR>select a.*&nbsp; from opvoucher a <BR>WHERE&nbsp; (a.customercode = @CustomerID) </P> <P>&nbsp;</P> <P>为何 前一句 where子句利用不到索引？</P> <P>&nbsp;</P>

Answer 1

不是这样写的啊（好像是这样的哦~你试下~~） select a.* from opvoucher a WHERE a.customercode = @CustomerID or a.customercode =@CustomerID is null

Answer 2

如果@CustomerID真的是NULL的话，会怎样？岂不是全选出来了？因为WHERE字句对于任何记录来说都为TRUE…… 也就是说，第一条SQL先别说什么索引了，本身就是错的！