In the book Prolog Programming for Artificial Intelligence, a large and intricate chapter is dedicated to Expert Systems. In these systems, a knowledge-database is represented through facts and rules in a declarative manner, and then we use the PROLOG inference engine to derive statements and decisions.
I was wondering: are there any examples of expert systems that represent knowledge through a standard Relational Database approach and then extract facts through SQL queries? Is there any research in this area? If not, why is a rule-based approach preferred?