[nendroid]

"attributes" are "generic" properties of relations.

So for example the union operator can take two different relations that have the same columns but different rows and create a new relation.

In this binary operation that takes two different relations.... the attributes of one relation (the columns) is indeed the attributes of the other relation.

See here: https://www.geeksforgeeks.org/basic-operators-in-relational-...

Look for "union" in the link above.

>Union on two relations R1 and R2 can only be computed if R1 and R2 are union compatible (These two relation should have same number of attributes and corresponding attributes in two relations have same domain)

It's the same thing for unary operations like select. A select expression can operate on different relations providing that the relations have the relevant attributes.