To get values that exist in one table that do not exist in another you want to do an anti-join. You can accomplish with using WHERE NOT IN or WHERE NOT EXISTS. My favorite is the EXCEPT set operator:
DECLARE @A TABLE (PPM_ID int);
DECLARE @B TABLE (SDLC int);
INSERT @A VALUES (1),(2),(3),(4);
INSERT @B VALUES (3),(4);
SELECT PPM_ID FROM @A
EXCEPT
SELECT SDLC FROM @B;
"I cant stress enough the importance of switching from a sequential files mindset to set-based thinking. After you make the switch, you can spend your time tuning and optimizing your queries instead of maintaining lengthy, poor-performing code."-- Itzik Ben-Gan 2001