Guy De Tré Rita M. M. De Caluwe Henri Prade Null Values Revisited in Prospect of Data Integration. 79-90 http://springerlink.metapress.com/openurl.asp?genre=article&issn=0302-9743&volume=3226&spage=79 2004 conf/icsnw/2004 ICSNW db/conf/icsnw/icsnw2004.html#TreCP04