title: formal query languages for secure relational databases. authors: marianne winslett , kenneth smith , xiaolei qian. venue: acm trans . database syst .. year: 1994
title: relational databases for querying xml documents : limitations and opportunities. authors: jayavel shanmugasundaram , kristin tufte , chun zhang , gang he , david j. dewitt , jeffrey f. naughton. venue: very large data bases. year: 1999