Question

Any explanation to the following queries :

Select x FROM y WHERE a = 1 OR a = 2  AND (b = 1 OR b = 2)

why it doesn’t return the correct info while this return the correct info :

Select x FROM y WHERE (a = 1 OR a = 2) AND (b = 1 OR b = 2)

Am i missing something here ?

      X   Y   (X OR Y)  X OR Y   


      1   0     1         1 
      0   1     1         1
      1   1     1         1
      0   0     0         0 

I know in term of precedence the () have priority , but why should i add them the the first part of the query ?

Correct me if I’m wrong

Thank you

Answer 1

AND has a higher precedence than OR so your first query is equivalent to this:

Select x FROM y WHERE a = 1 OR a = 3 OR (a = 2  AND (b = 1 OR b = 2))

Which is not equivalent to

Select x FROM y WHERE (a = 1 OR a = 2 OR a = 3) AND (b = 1 OR b = 2)

I guess you forgot the a = 3 part in your first query.

Operator precedence in MySQL