In SQL, what does using parentheses with an OR mean?

Question

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?

Answer 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