Ako sam dobro skapirao problem, možda ti nešto od ovoga pomaže:
SELECT * FROM Tasks WHERE Id IN (SELECT id_task FROM ass WHERE id_user = 1) OR Id NOT IN (SELECT id_task FROM ass)
SELECT * FROM Tasks LEFT JOIN ass ON Tasks.Id = ass.id_task
WHERE ass.id_user = 1 OR ass.id_user IS NULL
Pozdrav
ps. eh, ovo mi je prva poruka ovde, pa kako je red i običaj: POZDRAV SVIM FORUMASHIMA!
