As a sequel (not SQL) to relational SPARQL there's relational OWL, "In this paper we analyzed the similarities and differences between relational databases and description logics, in particular with respect to the role of schema constraints. Our analysis reveals more similarities than differences since, in both cases, constraints are just (restricted) first-order theories. Furthermore, reasoning about the schema is in both systems performed under standard first-order semantics, and it even employs closely related reasoning algorithms. The differences between relational databases and description logics become apparent only if one considers the problems related to reasoning about data. In relational databases, answering queries and constraint satisfaction checking correspond to model checking, whereas, in description logics they correspond to entailment problems."
Via All Problems Solved.