E. F. Codd's Rules

Dr. E. F. Codd's 12 rules

for defining a fully relational database

Note that based on these rules there is no fully relational database management system available today. In particular, rules 6, 9, 10, 11 and 12 are difficult to satisfy.

0. Foundation Rule
   A relational database management system must manage its stored data using only its relational capabilities.



1. Information Rule
   All information in the database should be represented in one and only one way - as values in a table.



2. Guaranteed Access Rule
   Each and every datum (atomic value) is guaranteed to be logically accessible by resorting to a combination of table name, primary key value and column name.



3. Systematic Treatment of Null Values
   Null values (distinct from empty character string or a string of blank characters and distinct from zero or any other number) are supported in the fully relational DBMS for representing missing information in a systematic way, independent of data type.



4. Dynamic On-line Catalog Based on the Relational Model
   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.



5. Comprehensive Data Sublanguage Rule
   A relational system may support several languages and various modes of terminal use. However, there must be at least one language whose statements are expressible, per some well-defined syntax, as character strings and whose ability to support all of the following is comprehensible:
1. data definition
2. view definition
3. data manipulation (interactive and by program)
4. integrity constraints
5. authorization
6. transaction boundaries (begin, commit, and rollback).


6. View Updating Rule
   All views that are theoretically updateable are also updateable by the system.



7. High-level Insert, Update, and Delete
   The capability of handling a base relation or a derived relation as a single operand applies nor only to the retrieval of data but also to the insertion, update, and deletion of data.



8. Physical Data Independence
   Application programs and terminal activities remain logically unimpaired whenever any changes are made in either storage representation or access methods.



9. Logical Data Independence
   Application programs and terminal activities remain logically unimpaired when information preserving changes of any kind that theoretically permit unimpairment are made to the base tables.



10. Integrity Independence
    Integrity constraints specific to a particular relational database must be definable in the relational data sublanguage and storable in the catalog, not in the application programs.



11. Distribution Independence
    The data manipulation sublanguage of a relational DBMS must enable application programs and terminal activities to remain logically unimpaired whether and whenever data are physically centralized or distributed.



12. Nonsubversion Rule
    If a relational system has or supports a low-level (single-record-at-a-time) language, that low-level language cannot be used to subvert or bypass the integrity rules or constraints expressed in the higher-level (multiple-records-at-a-time) relational language.