在SQL中，使用带有OR的括号是什么意思? [英] In SQL, what does using parentheses with an OR mean?

问题描述

示例:

select count(*) from my table
where
column1 is not null
and
(column1 = 4 OR column1 = 5)

示例2:

select count(*) from my table
where
column1 is not null
and
column1 = 4 OR column1 = 5

在具有真实列名的数据库中，我得到两个不同的结果.带有括号的是正确的，因为如果我这样做:

In my database with the real column names, I get two different results. The one with the parentheses is right because if I do:

select count(*) from my table
where
column1 is not null
and
column1 = 4

然后

select count(*) from my table
where
column1 is not null
and
column1 = 5

并将它们加在一起，我得到正确的答案...我认为.与上面带括号的第一个示例相同.

and add them together, I get the right answer...I think. Same as the first example with the parentheses above.

为什么通过更改OR测试的优先级会得到不同的结果?

Why do I get different results by changing precedence with the OR test?

推荐答案

它不是Oracle或SQL.这是基本的布尔逻辑. AND条件比OR条件更强"(具有优先级)，表示将首先对其求值:

It's not Oracle or SQL. It's basic boolean logic. The AND condition is "stronger" (has precedence) than OR, meaning it will be evaluated first:

column1 is not null
and
column1 = 4 OR column1 = 5

手段

column1 is not null
and
column1 = 4

首先评估

，然后在此和column1 = 5

添加括号可确保首先对OR求值，然后对AND求值.

Adding parentheses ensures OR is evaluated first and then the AND.

很像数学:

2 * 3 + 5 = 6 + 5 = 11

但是

2 * (3 + 5) = 2 * 8 = 16

在此处了解更多信息: http://msdn.microsoft.com/en-us/library/ms190276 .aspx

More reading here: http://msdn.microsoft.com/en-us/library/ms190276.aspx

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