Re: Relation Schemata vs. Relation Variables

From: paul c <toledobythesea_at_oohay.ac>
Date: Mon, 21 Aug 2006 15:19:08 GMT
Message-ID: <M9kGg.444334$IK3.95533@pd7tw1no>

Erwin wrote:

>> NOT EXISTS
>>  (SELECT SupplierId
>>   FROM Suppliers /* before */
>>   WHERE Status = 20
>>  EXCEPT
>>  SELECT SupplierId
>>   FROM Suppliers' /* after */) ;

>
> Is this an example of, to quote, "a relational expression in the
> general form of some join between t and t'." ????????????
>

It looks like it to me. In D&D terms, adding Dave P's apostophe, the SQL might become something like:

(Suppliers <AND> {Status 20} <AND> Suppliers') {} = TABLE_DUM;

I like the apostophe because it emphasizes that we are really comparing relations, not rows or what have you. Then from the dbms's point of view, we are not implementing time, as far as it is concerned, 'before' means 'this one' and 'after' means 'that one'.

p Received on Mon Aug 21 2006 - 10:19:08 CDT