SQL Server 与 NULL 比较

Question

我需要在一个值和它之前的值之间进行很多比较。

例如：ReceivedBy和PreviousReceivedBy。

我开始于：

WHERE ReceivedBy != PreviousReceivedBy

但是，如果其中一个值为 null，则返回 false，（当我真的需要它为 true 时）。所以我更新它看起来像这样：

WHERE ReceivedBy != PreviousReceivedBy
      OR (ReceivedBy IS NULL AND PreviousReceivedBy IS NOT NULL)
      OR (ReceivedBy IS NOT NULL AND PreviousReceivedBy IS NULL)

这工作正常，但我有大量需要比较的字段列表。我想找到一种方法来用更少的代码（不关闭ANSI_NULLS）进行这种比较。

显然，如果没有其他方法，那么我将把所有 3 行都放在一起进行比较。

更新：

举个例子，这是我希望的

ReceivedBy = 123  
PreviousReceivedBy = 123  
Result = FALSE  

ReceivedBy = 5  
PreviousReceivedBy = 123  
Result = TRUE  

ReceivedBy = NULL  
PreviousReceivedBy = 123  
Result = TRUE

ReceivedBy = 123  
PreviousReceivedBy = NULL  
Result = TRUE  

ReceivedBy = NULL  
PreviousReceivedBy = NULL  
Result = FALSE  

Answer 1

如果两列都是varchars，我会选择这样的内容：

coalesce(ReceivedBy, 'NULL') != coalesce(PreviousReceivedBy, 'NULL')

如果它们是整数，我会将一些值大大低于零（以明确表示null值）而不是'NULL'.

从列的名称来看，我认为它必须是字符串值还是整数值:)

更新

正如@Siyual 指出的，替换字符串应该“超出可能性范围”，您应该'NULL'用一些非字母字符替换上面的内容，如'#':)

Answer 2

在与可为空值进行比较时，我遇到了与您相同的问题，NULL总是返回unknown与我们期望的距离仅在TRUE或之间FALSE

我最终声明了一个带有这些逻辑的标量值函数，就像其他处理 null 的SQL一样

当任一输入为 null 时，普通比较运算符会产生 null（表示“未知”），而不是 true 或 false。例如，7 = NULL 产生 null，7 <> NULL 也是如此。当此行为不合适时，请使用 IS [ NOT ] DISTINCT FROM 结构：

a IS DISTINCT FROM b => a != b
a IS NOT DISTINCT FROM b => a == b

可以a IS NOT DISTINCT FROM b 重写为

(a IS NOT NULL AND b IS NOT NULL AND a=b) OR (a IS NULL AND b is NULL)

我使用sql_variant作为这些基本参数：int、datetime、varchar、...

create function IsEqual(
    @a sql_variant,
    @b sql_variant
)
returns bit
as
begin
    return (CASE WHEN (@a IS NOT NULL AND @b IS NOT NULL AND @a=@b) OR (@a IS NULL AND @b is NULL) THEN 1 ELSE 0 END);
end

create function IsNotEqual(
    @a sql_variant,
    @b sql_variant
)
returns bit
as
begin
    return 1-dbo.IsEqual(@a,@b);
end

使用

select dbo.IsEqual(null, null) Null_IsEqual_Null,
dbo.IsEqual(null, 1) Null_IsEqual_1,
dbo.IsEqual(1, null) _1_IsEqual_Null,
dbo.IsEqual(1, 1)  _1_IsEqual_1,
dbo.IsEqual(CAST('2017-08-25' AS datetime), null) Date_IsEqual_Null,
dbo.IsEqual(CAST('2017-08-25' AS datetime), CAST('2017-08-25' AS datetime)) Date_IsEqual_Date

对于您的情况

select dbo.IsNotEqual(123,123) _123_IsNotEqual_123,
dbo.IsNotEqual(5,123) _5_IsNotEqual_123,
dbo.IsNotEqual(Null,123) Null_IsNotEqual_123,
dbo.IsNotEqual(123,Null) _123_IsNotEqual_Null,
dbo.IsNotEqual(Null,Null) Null_IsNotEqual_Null