Re: Much Ado about Nothing

> As for duplicate tuples in a table being evil, I'm nodding my head to
> the point of spinal injury, but would this imply that duplicate tuples
> in all relations are evil, and aren't the result sets from SELECT also
> considered to be relations? ISTR this from relational algebra.

Duplicate tuples in a relation are impossible. If they are indistinct then they are the same tuple.

A relational "restrict", sometimes called "select", selects entire tuples, not just selected attributes. This is different from the SQL SELECT. Another relational operator, "project" eliminates rows, and also eliminates resulting duplication.

When the relational; operators, "restrict", "project", and "join" operate on relations, they always yield a relation. When the SQL SELECT operates on one or more tables, the result is always a table.

A table isn't always a relation. Not all tables represent relations.

--
Regards,
    David Cressey
    www.dcressey.com