Codd's definition of a relational database system

A set of qualitative criteria for the specification and evaluation of the EAGLET terminological database is provided by a simplified version of Codd's definition of a relational database system.

• All information must be represented explicitly in one and only one way: as values in tables.
• Each and every datum in the database must be accessible by specifying a table name, a column name, and a primary key.
• Null Values must be treated systematically.
•   At least one character-based language must be provided which can be used to modify the structure and contents of the database. This language must be able to do all of the following:
  • data definition
  • view definition
  • data manipulation
  • integrity constraints
• At least two integrity constraints need be supported:
  • No primary key can have a null value, and no primary key can be duplicated.
  • For every non-null foreign key there must exist a matching primary key.
• Although non-relational tools are allowed, it must be possible to manage the database using only relational tools, and the non-relational tools must not be allowed to bypass the integrity constraints imposed by the relational tools.