If you ever run across a situation that requires you to subtract two numbers and return the value if positive or 0 if negative, here are a couple of solutions.

1) Use IF: value = x - y; if value < 0 return 0; else return value;

2) Use UNION: select max(value) from ( select x - y as value union select 0 ) x

I like this way since IF requires plpgsql in Postgres whereas UNION can be used with plain sql.