Re: compound propositions

From: Bob Badour <>
Date: Mon, 15 Mar 2010 19:59:44 -0300
Message-ID: <4b9ebb69$0$12436$>

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.

> Am pondering the significance of the other comments.

Just remember: When you hear hooves, think horses. Received on Mon Mar 15 2010 - 23:59:44 CET

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