what is the equivalent relational algebra operator for not exists (in sql)? That is how do i represent not exists in relational algebra
A:
I think you're looking for the existential quantifier (∃), which you could then negate (~∃).
Response to comment: I don't remember most of my relational algebra, but if I was going to take a stab at it I would guess something along the lines of: σ∃σ(Y)(S). Or possibly π∃π(Y)(S); I don't quite remember if you'd want selection or projection for that.
eldarerathis
2010-09-17 18:52:34
if there is query like select * from S where exists(select * from Y); how can i write that in relational algebra
sudh
2010-09-17 20:22:11
@sudh: The comment section can't accurately display my response, so see the addendum to my answer.
eldarerathis
2010-09-17 20:47:00