The rules are 12, not 13. You probably have heard of Rule Zero and that is why you think the rules are 13.

"For any system that is claimed to be a relational database management system, that system must be able to manage data entirely through its relational capabilities."

If you check Codd’s 12 rules and apply them in a way that you give 1 point if the DB fulfils the rule, 1/2 point if the DB fulfils the rule partially and 0 point otherwise, then Oracle 8 is top on the market with 8.5 points.

It is funny that Oracle 7 fulfiled only 1 rule completely.