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From: davidm@cimshop.UUCP (David Masterson)
Newsgroups: comp.databases
Subject: Re: Relational Model
Message-ID: <445@cimshop.UUCP>
Date: 27 Jul 89 21:33:12 GMT
References: <AYknCfy00WB28IU6sk@andrew.cmu.edu> <18886@sequent.UUCP> <19101@sequent.UUCP> <172@bms-at.UUCP>
Organization: Consilium Mt. View CA
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In-reply-to: stuart@bms-at.UUCP's message of 26 Jul 89 18:23:42 GMT

>One thing in the relational model I disagree with: Duplicate records are
>useless in any order.  All tables should be required to have a "UNIQUE"
>combination of fields.  After all, we are dealing with *sets*, not *bags*. :-)
>
I think that is a requirement of the relational model (as defined by Codd).
What we thus far have is a tradeoff that most (all?) database vendors place on
the relational model for efficiency sake (can you imagine searching the
database on every update to determine if a uniqueness constraint is
violated?).  Besides, most allow you to put a unique index on the key and
enforce the constraint that way.

David Masterson
uunet!cimshop!davidm