A Review of Relational Concepts "The relational model includes an open-ended set of generic operators known collectively as the relational algebra [useful link] (the operators are generic because they apply to all possible relations, loosely speaking)...Each of the operators we discuss takes either one relation or two relations as operand(s) and returns another relation as result. Note: The—very important!—fact that the result is always another relation is referred to as the (relational) closure property. It is that property that, among other things, allows us to write nested relational expressions."
SQL doesn't support this and SPARQL has it as a feature (CONSTRUCT) but not a built-in property of SPARQL (results from a SELECT are not graphs). This is different to wanting transitive closure in SPARQL. It's interesting to see that OPTIONAL and UNION are still causing concerns for implementations.