What are Dr. Codd's 12 Rules for a Relational Database Model.

The most popular data storage model is the relational database, which grew from theseminal paper "A Relational Model of Data for Large Shared Data Banks," written by Dr. E. F. Codd in 1970. SQL evolved to service the concepts of the relational databasemodel. Dr. Codd defined 13 rules, oddly enough referred to as Codd's 12 Rules, for therelational model:

1. A relational DBMS must be able to manage databases entirely through its relational capabilities.
2. Information rule-- All information in a relational database (including tableand column names) is represented explicitly as values in tables.
3. Guaranteed access--Every value in a relational database is guaranteed to beaccessible by using a combination of the table name, primary key value, and column name.
4. Systematic null value support--The DBMS provides systematic support for the treatment of null values (unknown or inapplicable data), distinct from default values, and independent of any domain.
5. Active, online relational catalog--The description of the database and its contents is represented at the logical level as tables and can therefore be queried using the database language.
6. Comprehensive data sublanguage--At least one supported language must have a well-defined syntax and be comprehensive. It must support data definition,manipulation, integrity rules, authorization, and transactions.>
7. View updating rule--All views that are theoretically updatable can be updated through the system.
8. Set-level insertion, update, and deletion--The DBMS supports not only set level retrievals but also set-level inserts, updates, and deletes.
9. Physical data independence--Application programs and ad hoc programs arelogically unaffected when physical access methods or storage structures arealtered.
10. Logical data independence--Application programs and ad hoc programs are logically unaffected, to the extent possible, when changes are made to the table structures.
11. Integrity independence--The database language must be capable of defining integrity rules. They must be stored in the online catalog, and they cannot be by passed.
12. Distribution independence--Application programs and ad hoc requests are logically unaffected when data is first distributed or when it is redistributed.
13. Nonsubversion--It must not be possible to bypass the integrity rules defined through the database language by using lower-level languages.
    

This method has several advantages and many disadvantages. In its favor is the fact that the physical structure of data on a disk becomes unimportant. The programmer simply stores pointers to the next location, so data can be accessed in this manner. Also, data can be added and deleted easily. However, different groups of information could not be easily joined to form new information. The format of the data on the disk could not be arbitrarily changed after the database was created. Doing so would require the creation of a new database structure.Codd's idea for an RDBMS uses the mathematical concepts of relational algebra to break down data into sets and related common subsets.