Conditional inside WHERE clause

gistfile1.sql

-- Example of how to include an if conditional in a where clause

-- We only want rows where a = 1 or 2 but only a = 2 if b = 4
-- table structure: 
-- [ a | b ]
--  1   4
--  2   4
--  1   5
--  3   9
--  2   7
--  2   33

SELECT * FROM table 
 WHERE a IN (1,2) -- a is 1 or 2
 AND (a <> 2 OR a = 2 AND b = 4) -- a isn't 2 OR it is 2 and b is 4