Function in selection?

Hi, I want to make really advanced query and I'm not sure it's
possible... it would be "like that":

select A, B, C, D, E, F from TABLE_X ORDER BY D, C, E
  if A==0 AND B!=0 {A=B*Z.TABLE_Y}
  if A!=0 AND B==0 {B=A/Z.TABLE_Y}

OK, I don't really have problems with first line :), but how can I tell
him to select like the second and third line are telling it?
is that possible? :/

Tnx,

May