Codd's 12 rules was proposed by Edgar F. Codd, the pioneer of the relational model of databases. The 13 rules (from 0 to 12) that must be met to make the database management system relational ) Guidelines. The full relational system should fully support all the characteristics of the relational model. Edgar Cod, the founder of the relational model, specifically gave the basic principles that the whole relational system should follow.
Guidelines
A relational relational database system must be able to manage the database completely through its relational capabilities.
Principle 1: Information Standard
All information of a relational database system should be explicitly represented at the logical level using a method of table values.
Guideline 2: Guaranteed access guidelines
Rely on the combination of table name, master code and column name to ensure that each data item in the relational database can be accessed in a logical manner.
Principle 3: Systematic treatment of null values
The fully relational relational database system supports the concept of null values and uses a systematic method to handle null values.
Principle 4: Dynamic online data dictionary based on relational model
The description of the database is expressed in the same way as ordinary data at the logical level.
Principle 5: Unified data sub-language
A relational database system can have several languages and multiple terminal access methods, but there must be a language whose sentences can be expressed as strings of strict grammar, and can fully support various rules.
Guideline 6: View update guidelines
All theoretically updatable views should also be allowed to be updated by the system.
Principle 7: Advanced insert, modify and delete operations
The system should perform query optimization for various operations.
Principle 8: Physical independence of data
Regardless of any changes in the storage representation or access method of the database data, the application and terminal activities remain logically invariant.
Principle 9: Data logic independence
When any changes are made to the basic relationship where the theoretical information is not compromised, the application and terminal activities remain logically unchanged.
Principle 10: Data integrity and independence
The integrity constraints of relational databases must be defined in database language and stored in the data dictionary.
Principle 11: Distributed independence
Relational database systems keep the logic unchanged when introducing distributed data or redistributing data.
Guideline 12: Non-destructive criterion
If a relational database system has a low-level language, then this low-level language cannot violate or bypass integrity guidelines.