Hi Toon >> If SQL professionals would know about the underlying theories, then a lot (a >> real huge big lot) of FUD in our profession would disappear. Would you care to recommend a book for any SQL professionals who would like to know more about the maths underlying relational databases? ;-) Cheers, APC A P Clarke Software Architect Logica UK Public Sector Division Stephenson House 75 Hampstead Road London UK NW1 2PL Direct Tel: +44 (0)207 230 3160 Fax: +44 (0)207 446 1352 Email: andrew.clarke@xxxxxxxxxx This e-mail and any attachment is for authorised use by the intended recipient(s) only. It may contain proprietary material, confidential information and/or be subject to legal privilege. It should not be copied, disclosed to, retained or used by, any other party. If you are not an intended recipient then please promptly delete this e-mail and any attachment and all copies and inform the sender. Thank you.