1. Freebase: an open, shared database of the world’s knowledge.
freebase.com/
2. Google KG.
google.co.uk/insidesearch/features/search/knowledge.html
3. W3C: OWL 2 Web Ontology Language.
www.w3.org/TR/owl2-overview/
4. Abiteboul, S., Buneman, P., Suciu, D.: Data on the Web: From Relations to Semistructured Data and XML. Morgan Kaufmann, Burlington (1999)
5. Abiteboul, S., Hull, R., Vianu, V.: Foundations of Databases. Addison-Wesley, Boston (1995)