Semantics preserving SQL-to-SPARQL query translation for Nested Right and Left Outer Join

Despite the emergence of semantic web due to its high performance in managing a large amount of data through semantic filters, the relational databases are still the most used. Therefore establishing a connection between both heterogeneous systems becomes a relevant need so as to bridge the gap between them. Regarding the query mapping from relational world to semantic world (SQL-to-SPARQL), some solutions have been developed to realize this transformation but unfortunately, all existing approaches do not consider the semantic correspondence between Left/Right Outer Join command(s) and Optional pattern(s). This weakness has motivated us to investigate in this direction and suggest an efficient solution to deal with this problem by proposing, to the best of our knowledge, the first query mapping algorithm ensuring the semantic transformation of simple/nested Left/Right Outer Join command(s) in SQL queries to a simple/nested Optional pattern(s) in SPARQL equivalent ones without physical transformation of data.