Re: compound propositions
Date: Mon, 15 Mar 2010 19:59:44 -0300
paul c wrote:
> Bob Badour wrote:
>> paul c wrote: >> >>> Bob Badour wrote:
>>> not knowing its purpose any more than a dbms designer can predict the >>> exact purpose some unknown db is used for, other than to say >>> intersection and quantification are involved. If you mean >>> 'internal', that which an algebra operates with, it is basically the >>> expression 'Customer[id] join Client[id]', standing for the >>> intersection of the set of Customer id's that match Client id's and >>> vice-versa. >> >> Actually, for join it is "'id' is a customer and a client". For union, >> it would be "'id' is a customer or a client". >> ...
> Thanks. The nuance I'm detecting here is that there is some useful
> reason for phrasing an internal predicate in English rather than the
> more formal algebra.
No nuance at all. See Codd 1972: "Relational Completeness of Data Base Sublanguages" on the equivalent expressiveness of set algebra and predicate calculus.
You originally asked about a predicate expression using disjunction, which is the same as a union.
Just remember: When you hear hooves, think horses. Received on Mon Mar 15 2010 - 23:59:44 CET