3
Conjunctive Queries
•
The FO fragment consisting of:
R(x,y,…)
-- atomic formulas
x=y
-- equality
f
1
Æ
phi
2
-- conjunction
9
x.
f
-- existential quantifiers
•
•
Canonical form:
9
x
1
.
9
x
2
…
9
x
k
.(G
1
Æ
…
Æ
G
m
)
or, simply:
G
1
, …, G
m