Re: what is union?

From: paul c <>
Date: Sun, 13 Sep 2009 03:00:42 GMT
Message-ID: <ufZqm.43757$Db2.41180_at_edtnps83>

Mr. Scott wrote:
> "paul c" <> wrote in message 
> news:YSWqm.43749$Db2.18298_at_edtnps83...

>> Regarding so-called "union compatibility",.suppose relations A and B with
>> equal headings, say named h. Suppose non-empty relation U with only one
>> attribute that is not in any other heading.
>> value i): A Join B projected on h, D&D style =
>> (A <AND> B) {h}
>> value ii): A Union B (with unrestricted <OR> and Union restricted to equal
>> headings) =
>> {A <OR> B)
>> Every tuple in A and every tuple in B is in value i), no tuple in A or B
>> is not in value i), likewise for value ii).
> If A and B have the same heading, then you're wrong.  Suppose that no tuple 
> in A is also in B, then A Join B would be empty, as would A <AND> B <AND> U. 
Right you are, thanks. I was wondering about <rename> but left that op out. . Received on Sat Sep 12 2009 - 22:00:42 CDT

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