Codd applied rigorous mathematical theories, primarily set theory, to the management of data, and he compiled a list of criteria a database must meet to be considered relational. At its core, the relational database concept centers around storing data in tables. This concept is now so common as to seem trivial; however, not long ago designing a system capable of sustaining the relational model was considered a long shot with limited usefulness. Following are Codd's Twelve Principles of Relational Databases :
Information is represented logically in tables.
Data must be logically accessible by table, primary key, and column.
Null values must be uniformly treated as "missing information," not as empty strings, blanks, or zeros.
Metadata (data about the database) must be stored in the database just as regular data is.
A single language must be able to define data, views, integrity constraints, authorization, transactions, and data manipulation.
Views must show the updates of their base tables and vice versa.
A single operation must be available to do each of the following operations: retrieve data, insert data, update data, or delete data.
Batch and end-user operations are logically separate from physical storage and access methods.
Batch and end-user operations can change the database schema without having to recreate it or the applications built upon it.
Integrity constraints must be available and stored in the metadata, not in an application program. ...