The Challenge Of Optional Matching In Sparql

Conjunctive queries are arguably the most widely used querying mechanism in practice and the most intensively studied one in database theory. Answering a conjunctive query (CQ) comes down to matching all atoms of the CQ simultaneously into the database. As a consequence, a CQ fails to provide any answer if the pattern described by the query does not exactly match the data. CQs might thus be too restrictive as a querying mechanism for data on the web, which is considered as inherently incomplete. The semantic web query language SPARQL therefore contains the OPTIONAL operator as a crucial feature. It allows the user to formulate queries which try to match parts of the query over the data if available, but do not destroy answers of the remaining query otherwise. In this article, we have a closer look at this optional matching feature of SPARQL. More specifically, we will survey several results which have recently been obtained for an interesting fragment of SPARQL - the so-called well-designed SPARQL graph patterns.