3
Conjunctive Queries
•The FO fragment consisting of:

R(x,y,…)      -- atomic formulas
x=y               -- equality
f1 Æ phi2       -- conjunction
9 x.f             -- existential quantifiers
•
•Canonical form:
9 x1.9 x2… 9 xk.(G1 Æ … Æ Gm)
or, simply:  G1, …, Gm