[Skunkworks] SQL Help

Hello people, I have a query in the form; SELECT A, B, (C-D) AS E ... I need to return only values where E<0 for instance. I know i can achieve this in php, but is there a way i can achieve it at the SQL level? I have tried: SELECT A, B, (C-D) AS E WHERE E<0 but i get an "ORA-00904: : invalid identifier" error. E is not recognized. Any insights? Thank you Julianne