Path: utzoo!attcan!uunet!cimshop!davidm From: cimshop!davidm@uunet.UU.NET (David S. Masterson) Newsgroups: comp.databases Subject: Re: Outerjoin implementation? Message-ID: Date: 30 Sep 90 08:39:25 GMT References: <5803@plains.NoDak.edu> <30203@netnews.upenn.edu> Sender: davidm@cimshop.UUCP Distribution: comp Organization: Consilium Inc., Mountain View, California. Lines: 46 In-reply-to: aaron@grad2.cis.upenn.edu's message of 27 Sep 90 13:05:21 GMT In article <30203@netnews.upenn.edu> aaron@grad2.cis.upenn.edu (Aaron Watters) writes: >Nulls are certainly NOT a bogosity. As pointed out by many, they certainly >DO cause havoc in the relational paradigm -- but this indicates that >the relational paradigm is inadequate, NOT that nulls are ``bad.'' No, it indicates that current relational implementations are not complete, not that NULLs don't fit the paradigm. >PEOPLE [NAME, AGE] > [john, 24] > [jane, ??] > [mary, 13] > >Then what should we get if we select the people with age 24? > >TWO ANSWERS: >the `outer' answer and the `inner' answer > [john, 24] [john, 24] > [jane, 24] > >[similar grouping problem deleted] > >Roughly `outer' answers give `all possibilities' and `inner' >answers give `only that which is certain.' If you find this >intriguing at all, let me know. -aaron. This is better known as 3-valued logic which is a part of the relational model (as is 4-valued logic). Both answers (to both problems) are syntactically correct, the true answer is determined by the semantics of the problem (which you touched on at the end). To paraphrase Codd's terminology, is the first question: all people who definitely have an age = 24 or all people who MAYBE have an age = 24 ? BTW, do current OODBs treat 3 or 4 valued logic? I imagine they could, but do current implementations even address this? -- ==================================================================== 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!"