OPTIONAL and UNION cause some problems with SPARQL SPARQL queries are converted into Prolog or Datalog rules Three possible joins of tables with NULL values in OPTIONAL queries have been suggested C- Joins of Cautious Joins B- Joins or Brave Joins S-Joins Complexity of OPTIONAL/UNION is PSPACE (Perez et al., 2006) Translate CONSTRUCT queries into Datalog rules using non stratified negation http://con.fusion.at/dlvhex/