Computing Approximate Certain Answers over Incomplete Databases

Approximation Algorithms for Computing Certain Answers over Incomplete Databases

Certain answers are a widely accepted semantics of query answering over incomplete databases. Since their computation is a coNP-hard problem, recent research has focused on developing evaluation algorithms with correctness guarantees, that is, techniques computing a sound but possibly incomplete set of certain answers. In this paper, we show how novel evaluation algorithms with correctness guar...

Approximate Query Answering in Locally Closed Databases

The Closed-World Assumption (CWA) on databases expresses that an atom not in the database is false. A more appropriate assumption for databases that are sound but partially incomplete, is the Local ClosedWorld Assumption (LCWA), which is a local form of the CWA, expressing that the database is complete in a certain area, called the ‘window of expertise’. Databases consisting of a standard datab...

A Note on Computing Certain Answers to Queries over Incomplete Databases

How to Define Certain Answers

The standard way of answering queries over incomplete databases is to compute certain answers, defined as the intersection of query answers on all complete databases that the incomplete database represents. But is this universally accepted definition correct? We argue that this “one-size-fitsall” definition can often lead to counterintuitive or just plain wrong results, and propose an alternati...

Certain Answers as Objects and Knowledge

The standard way of answering queries over incomplete databases is to compute certain answers, defined as the intersection of query answers on all complete databases that the incomplete database represents. But is this universally accepted definition correct? We argue that this “one-size-fits-all” definition can often lead to counterintuitive or just plain wrong results, and propose an alternat...