what is union?

From: paul c <toledobythesea_at_oohay.ac>
Date: Sun, 13 Sep 2009 00:18:00 GMT
Message-ID: <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).

We could use the expression of value i) to "insert" to relvars, even define a union view. So what makes union different from projection of join? Received on Sat Sep 12 2009 - 19:18:00 CDT

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