The initial standard known as SQL1 is recorded in citeX3,ISO7. SQL2 was defined in [International Organization for Standardization, 1992]. Extensive presentations of SQL3 can be found in [Melton and Simon, 1993; Melton and Simon, 2002] and [Fortier, 1999]. Also, useful reference are [Line and Kline, 2000] and [J. Kauffman, 2001].