How exactly does using OR in a MySQL statement differ with/without parentheses?

Question

I have this query:

SELECT * FROM (`users`) WHERE `date_next_payment` <= '2011-02-02' 
    AND `status` = 'active' OR `status` = 'past due'

Which does not return the correct results. However, adding parentheses around the OR conditions makes it work like so:

SELECT * FROM (`users`) WHERE `date_next_payment` <= '2011-02-02'
    AND (`status` = 'active' OR `status` = 'past due')

My question is why is it different? I understand that's is considering the OR statement differently without the parentheses; but I don't understand how it's different.

I haven't found any docs that have been helpful on this. If there's any links out there I'd really appreciate it.

Answer 1

This is because OR has lower operator precedence than AND. Whenever the DB sees an expression like

A AND B OR C

the AND is evaluated first, i.e. it is equivalent to

(A AND B) OR C

So if you explicitly want

A AND (B OR C)

instead, you must put in the parentheses.

This is btw not specific to SQL. The order of precedence of these operators is the same in all programming languages I know (i.e. at least C, C++, C#, Java and Unix shell scripts).

Answer 2

SELECT * FROM (`users`) WHERE `date_next_payment` <= '2011-02-02' 
AND `status` = 'active' OR `status` = 'past due'

In this example, you will get all records where either

a) The date_next_payment is before 2nd Feb 2011 AND status is active

b) The status is past_due

So the past_due records will not be restricted by date.

SELECT * FROM (`users`) WHERE `date_next_payment` <= '2011-02-02'
AND (`status` = 'active' OR `status` = 'past due')

In this example, you will get all records where

a) The date_next_payment is before 2nd Feb 2011

AND

b) The status is either active or past_due

The brackets work like they do in maths or logic - the statements inside the brackets get evaluated first... so imagine seeing each step take place like this:

SELECT * FROM (`users`) WHERE `date_next_payment` <= '2011-02-02'
AND (`status` = 'active' OR `status` = 'past due')

So a record is encountered with a status of active...

SELECT * FROM (`users`) WHERE `date_next_payment` <= '2011-02-02'
AND (TRUE OR FALSE)

This then evaluates, with the OR condition... (TRUE OR FALSE) == TRUE

SELECT * FROM (`users`) WHERE `date_next_payment` <= '2011-02-02'
AND TRUE

And the date is 2011-01-01

SELECT * FROM (`users`) WHERE TRUE
AND TRUE

And finally, TRUE AND TRUE == TRUE

SELECT * FROM (`users`) WHERE TRUE

And so the record is returned...

Imagining your query being executed in steps like this against each row in the database sometimes helps to understand where to put your brackets.