Relational Calculus, which subset of FOL is supported

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 Received on Fri Jul 11 2008 - 15:54:43 CEST