Question

Gonna give an example since it’s hard to understand what i’m talking about:

   SELECT a.f1, 
          b.f1, 
          IF(a.f3 <> '', a.f3, b.f3) as f3 
     FROM a 
LEFT JOIN b 
       ON b.f4 = a.f4 
    WHERE f3 = 'something'

Now, this won’t work because i can’t have an alias as a field in the where clause, so how can I apply the condition to that f3 field that’s either from a or b. Preferably without putting a bunch of IFs in the where clause

Answer 1

Try this:

SELECT * FROM (
    SELECT a.f1, b.f1,
      IF(a.f3 <> '', a.f3, b.f3) as f3
    FROM a 
    LEFT JOIN b on b.f4 = a.f4
) S
WHERE S.f3 = 'something'

This is actually less efficient than replacing the value with the formula itself as it will require a full scan. To improve performance (but decrease readability) you can use this query:

SELECT a.f1 aF1, b.f1 bF1,
  IF(a.f3 <> '', a.f3, b.f3) as f3
FROM a 
LEFT JOIN b on b.f4 = a.f4
WHERE IF(a.f3 <> '', a.f3, b.f3) = 'something'