Rasqal 0.9.5 - SPARQL optionals "The one feature I particularly wanted to get out was optionals. These are pretty handy when you have a somewhat unknown graph that you are querying, which could have some useful triples, ahem, optionally. So rather than do lots of queries of the form "do you have triple (?x prop1 ?y) ?" and "do you have triple (?x prop2 ?z)?" they can be just attached to the other triples in the graph pattern, marked optional, and only if they match you get a binding, otherwise, the query carries on. This is all properly explained in the Including Optional Values section of the SPARQL query draft."
This (well something like it although different in a mathematical way) has been a long standing RFE in Kowari too.