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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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Codd's Twelve Rules - Rule 5 - Comprehensive Data Sublanguage Rule

Rule 5	Comprehensive Data Sublanguage Rule
Rule	A relational system may support several languages and various modes of terminal use. However, there must be 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, and transaction management operations (begin, commit, and rollback).
Description	This rule insist the existence of SQL like language to manipulate data. Manipulation should involve all the above said things (highlighted in blue color).
Example	Transact-SQL satisfies the manipulation in SQL Server, SQL satisfies the data manipulation in Oracle and MySQL support a subset of ANSI SQL.
Some DBMS that fulfills this property	T-SQL in SQL Server, SQL in Oracle, ANSI SQL in MySQL, SQL in IBM DB2

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Codd's Twelve Rules - Rule 6 - View Updating Rule

Rule 6	View Updating Rule
Rule	All views that are theoretically updateable are also updateable by the system.
Description	This rule deals with views, which are virtual tables used to give various users of a database different views of its structure. It’s one of the most challenging rules to implement in practice, and no commercial product fully satisfies it today. A view is theoretically updateable as long as it’s made up of columns that directly correspond to real table columns.
Example	All views are not updateable. Views can be updateable if they follow the certain rules unlike tables. The following syntax can be used to update views; UPDATE < view_name > SET<column1>=<value1>,<column2>=<value2>,..... WHERE <condition>;
Some DBMS that fulfills this property	SQL Server technically fulfills this rule by using INSTEAD OF triggers, MySQL support view update in some cases. For a view to be updatable, there must be a one-to-one relationship between the rows in the view and the rows in the underlying table in MySQL, All views that are theoretically updateable are also updateable by the Oracle system.

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Codd's Twelve Rules

Codd's Twelve Rules / Codd's Twelve Commandments / Relational Model Foundation Principles

Can we say “any database that supports SQL is a relational database”? No. At least, not completely. We need certain foundation principle to make this assessment. For any system to be called a relational database management system, the relational capabilities must be able to manage it completely. Edgar F. Codd, a pioneer of the relational model for databases, designed Codd’s twelve rules 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).

Those rules are listed below and the links will lead you to the description pages of those rules.