“A relational database stores information in a structured format called a schema. This schema is defined according the rules of database normalization. These rules are meant to ensure the integrity of the data. The schema for a database is broken up into the objects such as tables and constraints. Tables hold your data and are broken down into rows. each row represents a single entity such as a person and has columns which define the attributes of the entity such as age. Constraints define limitations around the data. For example a check constraint might limit the range of valid dates in a datetime column. From there queries can be run to extract data from the database. These queries will often join multiple tables to pull data from them.”

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- 08/19 Logical Symmetric Access, Data Sub-language, Kinds of Relations, Database Redundancy and Consistency, paper #2 in the new UNDERSTANDING THE REAL RDM series.
- 02/18 The Key to Relational Keys: A New Understanding, a new edition of paper #4 in the PRACTICAL DATABASE FOUNDATIONS series.
- 04/17 Interpretation and Representation of Database Relations, paper #1 in the new UNDERSTANDING THE REAL RDM series.
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The Misconceptions

• The data in a relational database is not physically stored in a schema -- a schema does not contain data.

• Explicit normalization (to 1NF) and further normalization (to 5NF) (which should not be confused) are necessary only to repair incorrectly designed databases, and data integrity -- as the term commonly understood -- is not ensured by the rules thereof.
• Tables, rows, and columns are not elements of a schema.
• SQL check constraints are only one type of constraints, are not the intended relational constraints, and SQL does not support all of those.
• Join is but one operation of the relational algebra (RA) that must be supported by a RDBMS.
• "Define limitations around the data" is an opaque description of what constraints do.

The Correct Answer

• A relational schema is a formal logical representation of a conceptual model of reality consisting of a collection of related entity groups (multigroup) -- each represented by database relations (not to be confused with mathematical relations) -- and relationships within and among the groups -- represented by semantic constraints[1]. Every database relation and the database as a whole are constrained to be consistent with the within- and among-groups relationships in the conceptual model[2]. Relations are sets of tuples -- representing facts about entities -- and have attributes -- representing properties in context -- defined on simple domains[3] -- representing properties[4].
• The schema (not to be confused with the graphical visualization thereof on some medium, which may not display all schema elements) does not contain user data. A relational database consists of the schema recorded in the catalog -- itself a system relational database of metadata -- and the data (tuples) recorded in the user database, both physically stored in any of a number of ways, independently of the logical schema[5].
• Relations can be visualized on some medium as R-tables, in which case tuples display as rows and attributes as columns, but these play no relational role[4].
• According to a new understanding, database relations are normalized (in 1NF) and fully normalized (in 5NF) by definition[6], otherwise they are not relations and the database is not relational[7].
• Database design must adhere to three fundamental principles:

The Principle of Expressive Completeness (POEC);

The Principle of Representational Parsimony (PORP);

The Principle of Orthogonal Design (POOD);

which jointly imply the Principle of Full Normalization (POFN), but not vice-versa[8]. This means that adherence to the three core principles implicity produces relational databases (i.e., consisting of relations) -- explicit application of the "rules of normalization" is necessary only to repair incorrectly designed databases due to failure to adhere to the three principles[9].