Re: what is union?
From: paul c <toledobythesea_at_oohay.ac>
Date: Sun, 13 Sep 2009 03:00:42 GMT
Message-ID: <ufZqm.43757$Db2.41180_at_edtnps83>
>> Regarding so-called "union compatibility",.suppose relations A and B with
>> equal headings, say named h. Suppose non-empty relation U with only one
>> attribute that is not in any other heading.
>>
>> value i): A Join B projected on h, D&D style =
>> (A <AND> B) {h}
>>
>> value ii): A Union B (with unrestricted <OR> and Union restricted to equal
>> headings) =
>> {A <OR> B)
>>
>> Every tuple in A and every tuple in B is in value i), no tuple in A or B
>> is not in value i), likewise for value ii).
Right you are, thanks. I was wondering about <rename> but left that op out. . Received on Sun Sep 13 2009 - 05:00:42 CEST
Date: Sun, 13 Sep 2009 03:00:42 GMT
Message-ID: <ufZqm.43757$Db2.41180_at_edtnps83>
Mr. Scott wrote:
> "paul c" <toledobythesea_at_oohay.ac> 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 Sun Sep 13 2009 - 05:00:42 CEST