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.

tucekjust what i was looking for. thank you!