1. Universality of data retrieval language;Aho,1979
2. QBD*: A graphical query language with recursion;Angelaccio;Technical Report N.31.88 of the Dipartimento di Informatica e Sistemistica, Universita' di Roma «La Sapienza,1988
3. QBD*: A graphical query language with recursion;Angelaccio,1989
4. Design of statistical databases: a methodology for the conceptual Step;Batini;Information Systems,1988