Vasilis Aggelis Stavros S. Cosmadakis Optimization of Nested SQL Queries by Tableau Equivalence. 31-42 http://link.springer.de/link/service/series/0558/bibs/1949/19490031.htm 1999 conf/dbpl/1999 DBPL db/conf/dbpl/dbpl1999.html#AggelisC99