Codd's Twelve Rules - Rule 4 - Dynamic Online Catalog Based on the Relational Model

Codd's Twelve Rules - Rule 4 - Dynamic Online Catalog Based on the Relational Model

Rule 4	Dynamic Online Catalog Based on the Relational Model
Rule	The database description is represented at the logical level in the same way as ordinary data, so authorized users can apply the same relational language to its interrogation as they apply to regular data.
Description	This rule requires that a relational database be self-describing. In other words, the database must contain certain system tables whose columns describe the structure of the database itself, or alternatively, the database description is contained in user-accessible tables. In other words, this rule insists a data dictionary that stores meta data.
Example	INFORMATION_SCHEMA system views in SQL Server,   INFORMATION_SCHEMA  information database in MySQL, and Oracle Data dictionary resides in SYSTEM tablespace and can be accessed using the prefixes USER, ALL, or DBA.
Some DBMS that fulfills this property	SQL Server, MySQL, and Oracle.

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