what is union?

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 Sun Sep 13 2009 - 02:18:00 CEST