"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

1. Database truth of the week

"Within the database field, it is common to refer to three “level” of description: conceptual, logical, and physical. Both the logical level and the physical level are formal systems. By contrast, the conceptual level is typically an informal system and refers to the subject of the database.

The conceptual language is a subject language, in the terminology of formal systems. The conceptual level identifies the concepts to be formally represented by the logical and physical levels, and how users think and talk about those concepts. This level corresponds only informally to the so-called “conceptual schema” of earlier approaches to information management, which emphasized the capture of conceptual information using various techniques including diagrams and documentation having various degrees of formality, but not forming a strictly formal system themselves." -- David McGoveran

2. What's wrong with this database picture?

"Can you have 2 tables, VIEWS and DOWNLOADS, with identical structure in a good DB schema (item_id, user_id, time). Some of the records will be identical but their meaning will be different depending on which table they are in. The "views" table is updated any time a user views an item for the first time. The "downloads" table is updated any time a user downloads an item for the first time. Both of the tables can exist without the other ..."

"I have a database for a school ... [with] are numerous tables obviously but consider these:

CONTACT - all contacts (students, faculty) has fields such as LAST, FIRST, MI, ADDR, CITY, STATE, ZIP, EMAIL;
FACULTY - hire info, login/password for electronic timesheet login, foreign key to CONTACT;
STUDENT - medical comments, current grade, foreign key to CONTACT.

Do you think it is a good idea to have a single table hold such info? Or, would you have had the tables FACULTY and STUDENT store LAST, FIRST, ADDR and other fields? ..."

"Per Date’s AN INTRODUCTION TO DATABASE SYSTEMS, Date & Darwen’s DATABASES, TYPES, AND THE RELATIONAL MODEL, and related references, the features of a relational database are values, types, attributes, tuples, relations, relation-valued variables, operators, and constraints.

• A type is a set of values and related operators.
• An attribute is a name, value, type triple.
• A tuple is a set of attributes.
• A relation is a set of tuples with a given heading.
• A relation-valued variable (known as a relvar) is a persistent variable whose time-varying value is a relation." --Dave Voorhis, Computer scientist; lead developer of Rel, a true relational database system, Quora.com

1. Database Truth of the Week

"A formal system is a systematic way of representing something. We call that something the subject and often refer to it as the “subject system”, although it might not be “systematic” in any sense at all. By contrast we call the formal system the object system -- a system of abstract “objects”.

Representing a subject with a formal system allows us to reason about it without getting trapped in ambiguities, or circular arguments. The formal system becomes a theory about that portion of the subject that has been represented and when that portion is faithfully represented by the theory, we say that portion is a model of the theory.There are at least three distinct uses of languages necessary to use or apply any formal system that correspond to the three distinct ways in which we need to discuss and use formal systems. It is standard to refer to these three uses by different names and as if they were distinct languages: subject, object and meta-language." --David McGoveran

2. What's Wrong With This Database Picture?

"Per Date’s AN INTRODUCTION TO DATABASE SYSTEMS, Date & Darwen’s DATABASES, TYPES AND THE RELATIONAL MODEL and related references, the features of a relational database are values, types, attributes, tuples, relations, relation-valued variables, operators, and constraints.

• A type is a set of values and related operators.
• An attribute is a name, value, type triple.
• A tuple is a set of attributes.
• A relation is a set of tuples with a given heading.
• A relation-valued variable (known as a relvar) is a persistent variable whose time-varying value is a relation." --Dave Voorhis (Computer scientist; lead developer of Rel, a true relational database system), --What are the features of a relational database?, Quora.com

"In SQL RDBMSes (such as MS SQL Server and Oracle] tables are permently stored relations, where the column names defined in the data dictionary form the "heading" and the rows are the "tuples" of the relation."

"A relation can be represented by a table in database. A relation in the context of modeling a problem will include the fields and possibly the identification of fields which have relationships with other relations..."

"Put simply, a "relation" is a table, the heading being the definition of the structure and the rows being the data."

"In simple English: relation is data in tabular format with fixed number of columns and data type of each column. This can be a table, a view, a result of a subquery or a function etc."

"A relation is a table, which is a set of data. A table is the result of a query."
--What is a relation in database terminology?, StackOverflow.com