DIFFERENCE

This operator, proposed in Gyssens & Lakshamanan (1997), Pedersen & Jensen (1999), and Pedersen, Jensen, & Dyreson (2001), makes it possible, given two MADs with common schemas, to take the set difference of the facts (the dimensions of the first argument MAD are retained) and to restrict the fact-dimension relations to the new fact set. Note that we do not take the set difference of the dimensions because this does not make sense. This operator is very similar to a previous operator, called set difference, proposed in Ozsoyoglu, Ozsoyoglu, & Matos (1997).