1. Rule of Foundation: A relational database management system (RDBMS) must have a solid foundation based on a well-defined data structure. This means that data should be organized in tables with rows and columns.
Example: Let's say we have a database for a school. We can have a table called "Students" with columns like "Student ID," "Name," and "Grade."
2. Rule of Guaranteed Access: Every individual data item in the database should be accessible using a combination of the table name, primary key, and column name. This ensures reliable and precise access to data.
Example: In our "Students" table, each student has a unique "Student ID" as the primary key. We can access a specific student's name using their ID.
3. Rule of Systematic Treatment of Null Values: Null values should be supported in the database system, allowing missing or unknown data to be represented accurately.
Example: Suppose we have a "Students" table with a column called "Date of Birth." If the date of birth for a particular student is unknown, we can store a null value in that column.
4. Rule of Dynamic Online Catalog: The database system should have a catalog that contains metadata about the database, including information about tables, columns, and relationships. This catalog should be accessible and updatable by authorized users.
Example: The catalog would include information like the names of tables, the columns they contain, and the data types of those columns.
5. Rule of Comprehensive Data Sublanguage: The database system should have a comprehensive and consistent language for defining, manipulating, and querying data. This language should be separate from any programming language used to build applications.
Example: SQL (Structured Query Language) is a common data sublanguage used in relational database systems.
6. Rule of View Updating: The database system should support updating views (virtual tables derived from other tables or views) as well as base tables. This ensures that changes made to views are properly reflected in the underlying tables.
Example: Suppose we have a view called "TopStudents" that shows the names of the top-performing students. If a new student achieves a high score, the "TopStudents" view should be updated automatically.
7. Rule of High-Level Insert, Update, and Delete: The database system should support high-level operations like inserting, updating, and deleting entire records, rather than requiring users to manipulate data at the individual attribute level.
Example: Instead of updating each attribute separately, we can use a single command to update the entire record for a student, including their name, grade, and any other relevant information.
8. Rule of Physical Data Independence: The database system should be independent of the physical storage structures and access methods used to store data. This allows changes to the physical storage without affecting the logical representation of the data.
Example: We can change the underlying storage technology (e.g., from hard disk drives to solid-state drives) without altering the way users interact with the database.
9. Rule of Logical Data Independence: The database system should be independent of the logical structure of the data, allowing changes to the logical schema without affecting the applications that use the database.
Example: We can add new columns or tables to the database schema without needing to modify all the existing queries or applications that rely on the database.
10. Rule of Integrity Independence: The database system should provide mechanisms to enforce integrity constraints (e.g., primary keys, foreign keys, uniqueness) independently of application programs. This ensures data consistency and prevents invalid data from being stored in the database.
Example: If we define a primary key constraint on the "Student ID" column in the "Students" table, the database system will automatically enforce the uniqueness of student IDs, regardless of the application code.
11. Rule of Distribution Independence: The database system should be able to distribute the database across multiple locations or systems without changing the application's ability to access and manipulate the data.
Example: A distributed database can have multiple servers in different locations, but applications can still interact with the database as if it were a single, centralized system.
12. Rule of Non-Subversion: The database system should not allow users to bypass or subvert the security and integrity features provided by the system. This ensures that the database remains secure and reliable.
Example: The database system should prevent unauthorized users from directly modifying or accessing data without proper authentication and authorization.
These are the 12 rules proposed by E.F. Codd to define the characteristics of a relational database management system (RDBMS) that ensures data integrity, consistency, and usability. By adhering to these rules, database systems can provide reliable and efficient storage and retrieval of data.
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