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declare @OrderNo varchar(20)
set @OrderNo = ‘123’ + null
print @OrderNo
结果为:NULL 什么也没有
即: 字符串 + NULL = NULL (此点要注意!)
换用 set @OrderNo = ‘123’ + ISNULL(@SDF, ”)
NULL 与任何操作都是NULL ,无论是 加法还是逻辑运算