International Journal of Web Applications BAHAJ Mohamed, Soussi Nassima 9 4 2017 http://www.dline.info/ijwa/fulltext/v9n4/ijwav9n4_3.pdf Despite the emergence of the Semantic Web and its performance, relational databases are still the most used for data manipulation, making the mapping of the relational world to the semantic world a very pertinent research topic to fill the gap between these two heterogeneous systems and ensure better semantic interoperability without physical transformation of data represented with different syntaxes. Regarding the transformation of SQL queries to SPARQL queries semantically equivalent, several algorithms have been developed to ensure this conversion; unfortunately, most of these algorithms have shortcomings and weaknesses in the consideration of the Left Outer Join commands in SQL queries. This problem has motivated us to write this paper so as to remedy this lack. Our main contribution is as follows: (a) establish an effective semantic transformation algorithm for Left Outer Join commands (in their simple and nested structures) to their equivalent in the SPARQL language, (b) integrate this result to RETRO (A Framework for Semantics Preserving SQL-to-SPARQL Translation) in order to have a richer algorithm that contribute to treatment of a maximum of SQL commands.