where子句中的按位运算符 [英] Bitwise Operators in a where clause

问题描述

我对按位运算符是完全陌生的，并且我无法确定数据库中的值是否可以正确计算出.

I am completely new to bitwise operators and I having trouble figuring out values in a database are calculating out correctly.

INSERT INTO @ObjectCounts (CountOf, Server, Total)
(SELECT     'RandomString', CAST(ServerGUID AS varchar(40)), COUNT(*) AS Num
FROM         TableName
WHERE     (Flags & (8 | 16) = 16)

在Flags列中将值写为"2359624"，"35160"，"2360072"给出了一些示例，而where子句让我感到困惑.

我在Google上找到的示例将两个值的数字转换为二进制，然后将它们加在一起?无论如何，由于不了解按位运算的工作原理，我不知道该运算符的值将变成16.

The where clause is what is getting me as the values in the Flags column are written out as "2359624", "35160", "2360072" to give a few examples.

The examples I found on google are translating the numbers of two values into binary and then adding them together? In any case, I don''t know how the value of this operator would turn into 16 because I don''t understand how the bitwise operation works.

推荐答案

在查看按位运算符时，首先要做的是停止查看十进制数字！而是将它们翻译为十六进制:

When looking at bit wise operators, the first thing to do is stop looking at numbers in decimal! Translate them to Hex instead:

  Dec         Hex
2359624 -> 0x240148
  35160 -> 0x008958
2360072 -> 0x240308
      8 -> 0x000008
     16 -> 0x000010

现在就说得通了！

Now it makes sense!

WHERE Flags & (8 | 16) = 16)

翻译为:

WHERE bit 3 of Flags is zero AND bit 4 of Flags is one

，原因是:

Because:

8 | 16      ->   0x08 | 0x10     ->   0x18

然后

Flags & (8 | 16)    ->    Flags & 0x18

仅返回标志的原始Bit 3和Bit 4值-所有其他位都被AND运算符丢弃.
最后，将该值与16(或0x10)进行比较，因此它希望第4位打开，第3位关闭.
您提供给我们的所有Flags 示例都没有通过该测试！

Which returns only the original Bit 3 and Bit 4 values of Flags - all other bits are discarded by the AND operator.
Finally, that value is compared against 16 (or 0x10) so it wants bit 4 on, and bit 3 off.
And none of the Flags examples you gave us passes that test!

如果我理解您想找出flags列包含8还是16(使用位计算).如果正确，请分别进行比较.例如:

If I understood you want to find out if the flags column contains 8 or 16 (using bit calculations). If that''s correct, compare them separately. For example:

... WHERE FLAGS & 8 = 8 OR FLAGS & 16 = 16

例如，请考虑以下内容:

For example consider following:

select 31 & 16

结果是16和

Result is 16 and

select 31 & 8

结果是8
和

Result is 8
and

select 32 & 8

结果为0

result is 0

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