Re: choice of character for relational division

From: Bob Badour <bbadour_at_pei.sympatico.ca>
Date: Sat, 31 Mar 2007 21:47:14 GMT
Message-ID: <CFAPh.17837$PV3.184014@ursa-nb00s0.nbnet.nb.ca>

Marshall wrote:

> On Mar 31, 9:08 am, Bob Badour <bbad..._at_pei.sympatico.ca> wrote:
> 

>>Marshall wrote:
>>
>>>Let us consider the choice of characters to use for
>>>relational operators. It might be desirable to use
>>>different characters for the relational operators from
>>>the scalar ones, so we avoid using * for join, even
>>>though it is in some sense a product operator.
>>
>>>Set subtraction already has a standard character
>>>in common usage: \
>>
>>>But we also need a character for relational division.
>>>The / character is often used, but that's the same
>>>as numerical division. Bummer.
>>
>>>So, if you had to choose an ascii character for
>>>relational division, which one would you use
>>>and why?
>>
>>Does a unique relational "division" exist?

> 
> Well, I think so. It's in TTM, no? Suppliers who supply all parts?
> That sort of thing?

One needs to have set equality and set containment operators. If one has those, the query seems simple. What does a special operator for it buy one?

>>If "product" has a completely
>>different name "join", why should a "divide" use the same name?

> 
> A fair question. The inverse of union is called "set subtraction"
> rather than just "subtraction." Maybe call it "inverse join?"
> "Unjoin?" "Relational division?"

Can you describe the full operation? Maybe show an inverse for a join that is not a cartesian product?

>>If we have an equality operation for relations, does a "divide"
>>operation give us all that much?

>
> I believe it does.

Belief is for sundays. What does a "divide" give? Received on Sat Mar 31 2007 - 16:47:14 CDT