This database design was inspired by a blog post called Reflections on NULLs in Response to “Much Ado About Nothing” by van Wijk which referred to Much Ado About Nothing .
Bibliography
Darwen, H. (2006) How To Handle Missing Information Without Using NULL , http://www.dcs.warwick.ac.uk/%7Ehugh/TTM/Missing-info-without-nulls.pdf , viewed 30 May 2012.
de Haan, L. & Koppelaars, T. (2007) Applied Mathematics for Database Professionals , Apress.
ISO/IEC (2002) Information technology — Z formal specification notation — Syntax, type system and semantics , ISO/IEC 13568.
Lightfoot, D. (2001) Formal Specification Using Z , Palgrave.