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The following posts have been re-written to bring in line with the McGoveran formalization and interpretation of Codd's true RDM. Re-reading is strongly recommended.

"But the core Information Principle (IP) of the RDM mandates that all information in a relational database be represented explicitly and in exactly one way -- as values of relation attributes defined on domains. The difference between relation names is, thus, meaningful information, the representation of which violates the IP and the RDM, for which reason it is inaccessible to the DBMS: consider the candidate tuple {v1,v2} -- it is impossible for the DBMS to know to which relation it belongs based on the relation and attribute names because it does not understand semantics!" Database Design Relation Predicates and “Identical Relations”

"Some set defining properties are formed as the disjunction of two or more properties (a kind of relationship between two common properties). These disjuncts, taken together, are meaning criteria. Each meaning criterion (an individual disjunct) induces a partitioning of a set into two subsets, those that meet the criterion and those that do not. Alternatively, we can say that each meaning criterion serves to differentiate a possible subset of a set from other subsets of the set (some of the possible subsets will be disjoint, while others not). Each of the possible subsets of the set is then defined by (“inherits”): The defining properties of the set conjoined with at least one meaning criterion (that or those becoming the defining property, or properties, respectively, specific to the proper subset)." Meaning Criteria and Entity Supertype-Subtypes

"Although they are no longer used, inquiries about them persist and with the current proliferation of non-relational products (e.g., NoSQL, graph DBMSs) there is value in understanding them. The closest the industry came to implementing the RDM is SQL which, despite its poor relational fidelity, proved much superior relative to the complexity and inflexibility of preceding DBMSs. But the rules still expose poor relational fidelity of SQL DBMS's that have not been addressed for four decades, while new RDM violations were introduced.

We offer here our clarifications on the rules. For each rule, we:

• Explain its intended objective;
• Offer clarifications, some of which reflect our current understanding of the RDM -- distinct from conventional wisdom -- based on its dual theoretical foundation and a careful analysis of Codd's work;" --Interpreting Codd's 12 Rules