Codd's 12 rules

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Codd's twelve rules are a set of thirteen rules (numbered zero to twelve) proposed by Edgar F. Codd, a pioneer of the relational model for databases.

Description

The rules are designed to define what is required from a database management system in order for it to be considered relational, i.e., a relational database management system (RDBMS).

They are sometimes jokingly referred to as "Codd's Twelve Commandments".

The Rules

Rule 0: The Foundation rule

A relational database management system must manage its stored data using only its relational capabilities.

The system must qualify as relational, as a database, and as a management system.

For a system to qualify as a relational database management system (RDBMS), that system must use its relational facilities (exclusively) to manage the database.

Rule 1: The information rule

All information in a relational database (including table and column names) is represented in only one way, namely as a value in a table.

Rule 2: The guaranteed access rule

All data must be accessible. This rule is essentially a restatement of the fundamental requirement for primary keys.

It says that every individual scalar value in the database must be logically addressable by specifying the name of the containing table, the name of the containing column and the primary key value of the containing row.

Rule 3: Systematic treatment of null values

The DBMS must allow each field to remain null (or empty). Specifically, it must support a representation of "missing information and inapplicable information" that is systematic, distinct from all regular values (for example, "distinct from zero or any other number", in the case of numeric values), and independent of data type.

It is also implied that such representations must be manipulated by the DBMS in a systematic way.

Rule 4: Active online catalog based on the relational model

The system must support an online, inline, relational catalog that is accessible to authorized users by means of their regular query language. That is, users must be able to access the database's structure (catalog) using the same query language that they use to access the database's data.

Rule 5: The comprehensive data sublanguage rule

The system must support at least one relational language that

  • Has a linear syntax
  • Can be used both interactively and within application programs
  • Supports:
    • Data definition operations (including view definitions)
    • Data manipulation operations (update as well as retrieval)
    • Security and integrity constraints
    • Transaction management operations (begin, commit, and rollback)

Rule 6: The view updating rule

All views that are theoretically updatable must be updatable by the system.

Rule 7: High-level insert, update, and delete

The system must support set-at-a-time insert, update, and delete operators. This means that data can be retrieved from a relational database in sets constructed of data from multiple rows and/or multiple tables. This rule states that insert, update, and delete operations should be supported for any retrievable set rather than just for a single row in a single table.

Rule 8: Physical data independence

Changes to the physical level (how the data is stored, whether in arrays or linked lists etc.) must not require a change to an application based on the structure.

Rule 9: Logical data independence

Changes to the logical level (tables, columns, rows, and so on) must not require a change to an application based on the structure. Logical data independence is more difficult to achieve than physical data independence.

Rule 10: Integrity independence

Integrity constraints must be specified separately from application programs and stored in the catalog. It must be possible to change such constraints as and when appropriate without unnecessarily affecting existing applications.

Rule 11: Distribution independence

The distribution of portions of the database to various locations should be invisible to users of the database. Existing applications should continue to operate successfully:

  • when a distributed version of the DBMS is first introduced; and
  • when existing distributed data are redistributed around the system.

Rule 12: The nonsubversion rule

If the system provides a low-level (record-at-a-time) interface, then that interface cannot be used to subvert the system, for example, bypassing a relational security or integrity constraint.

See also

External links

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