Path: utzoo!attcan!uunet!cimshop!davidm From: cimshop!davidm@uunet.UU.NET (David S. Masterson) Newsgroups: comp.databases Subject: Re: Outerjoin implementation? (4-valued question) Message-ID: Date: 8 Oct 90 03:35:06 GMT References: <5803@plains.NoDak.edu> <30203@netnews.upenn.edu> <152008@felix.UUCP> Sender: davidm@cimshop.UUCP Distribution: comp Organization: Consilium Inc., Mountain View, California. Lines: 29 In-reply-to: asylvain@felix.UUCP's message of 4 Oct 90 23:17:54 GMT In article <152008@felix.UUCP> asylvain@felix.UUCP (Alvin E. Sylvain) writes: Whoa, wait a minute! Can you explain to this meager humble dum-dum what is 4 valued logic? If this is explained much deeply in some periodic list of FAQ that I failed to locate, please to forgive! Hehe, yea, it threw me for awhile when I first saw it in Codd's new book (its still a bit confusing, but I really haven't studied it). Basically, as I understand it, it breaks down like this: 2-valued logic: defines the results of applying relational operators to TRUE or FALSE operands. 3-valued logic: same as 2-valued logic except operands might be TRUE, FALSE, or UNKNOWN. 4-valued logic: same as 3-valued logic except operands might be TRUE, FALSE, NOT-APPLICABLE, UNKNOWN-BUT-APPLICABLE (forget if that's the right terminology). Check Codd's book, "The Relational Model for Database Management Version 2" for more in-depth reasoning on 4-valued logic's applicability to relational databases. -- ==================================================================== David Masterson Consilium, Inc. uunet!cimshop!davidm Mtn. View, CA 94043 ==================================================================== "If someone thinks they know what I said, then I didn't say it!"