问题
Example:
select count(*) from my table
where
column1 is not null
and
(column1 = 4 OR column1 = 5)
Example 2:
select count(*) from my table
where
column1 is not null
and
column1 = 4 OR column1 = 5
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
and then
select count(*) from my table
where
column1 is not null
and
column1 = 5
and add them together, I get the right answer...I think. Same as the first example with the parentheses above.
Why do I get different results by changing precedence with the OR test?
回答1:
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
Means
column1 is not null
and
column1 = 4
is evaluated first, then OR is applied between this and column1 = 5
Adding parentheses ensures OR is evaluated first and then the AND.
Pretty much like in maths:
2 * 3 + 5 = 6 + 5 = 11
but
2 * (3 + 5) = 2 * 8 = 16
More reading here: http://msdn.microsoft.com/en-us/library/ms190276.aspx
回答2:
This comes down to whether your expression is parsed as:
(column1 is not null and column1 = 4) OR column1 = 5
or
column1 is not null and (column1 = 4 OR column1 = 5)
See the difference?
来源：https://stackoverflow.com/questions/10034489/in-sql-what-does-using-parentheses-with-an-or-mean