  In this paper, we study consistent query answering in tables with nulls and
functional dependencies. Given such a table T, we consider the set Tuples of
all tuples that can be built up from constants appearing in T, and we use set
theoretic semantics for tuples and functional dependencies to characterize the
tuples of Tuples in two orthogonal ways: first as true or false tuples, and
then as consistent or inconsistent tuples. Queries are issued against T and
evaluated in Tuples. In this setting, we consider a query Q: select X from T
where Condition over T and define its consistent answer to be the set of tuples
x in Tuples such that: x is a true and consistent tuple with schema X and there
exists a true super-tuple t of x in Tuples satisfying the condition. We show
that, depending on the status that the super-tuple t has in Tuples, there are
different types of consistent answer to Q. The main contributions of the paper
are: (a) a novel approach to consistent query answering not using table
repairs; (b) polynomial algorithms for computing the sets of true-false tuples
and the sets of consistent-inconsistent tuples of Tuples; (c) polynomial
algorithms in the size of T for computing different types of consistent answer
for both conjunctive and disjunctive queries; and (d) a detailed discussion of
the differences between our approach and the approaches using table repairs.
