Hi all!

It is often stated that relational calculus (tuple domain calculus) is
a subset of first order logic.
In the dbforum, wikipedia, wapedia articles:
- http://en.wikipedia.org/wiki/Relational_algebra
- http://wapedia.mobi/en/Relational_algebra
- http://www.dbforums.com/archive/index.php/t-424876.html
it is claimed that:
1) "Codd's algebra is not in fact complete with respect to first-order
logic, he restricted the operands to finite relations only and also
proposed restricted support for negation (NOT) and disjunction (OR)"
2) "Relational algebra actually corresponds to a subset of first-order
logic that is Horn clauses without recursion and negation."
3) ">What is the relation between the relational model and first oder
logic? They are one and the same."

Which "subset" of FOL is supported by the relational calculus in
detail?
I know Codd's (original) paper from 1970 (unfortunately I don't have
the original one from 1969), but I wasn't able to find an answer to my
question.

I'm looking for a more "serious comparison" of relational algebra (or
relational calculus) and FOL (or predicate logic in general), i.e.
expressive power, limitations, etc.

I really appreciate your help.


Best regards,
Tom