In October 1985, Codd presented 12 rules that a database must obey if it is to be considered truly relational:
- Information rule — All information in a relational database is represented explicitly at the logical level and in exactly one way — by values in tables.
- Guaranteed access rule — Each and every datum (atomic value) in a relational database is guaranteed to be logically accessible by resorting to a combination of table name, primary key value, and column name.
- Systematic treatment of NULL values — NULL values are supported in a fully relational DBMS for representing missing information and inapplicable information in a systematic way, independent of the data type.
- Dynamic online catalog based on the relational model — The database description is represented at the logical level in the same way as ordinary data, so that authorized users can apply the same relational language to its interrogation as they apply to the regular data.
- Comprehensive data sublanguage rule — A relational system may
support several languages and various modes of terminal use (for example, the
fill-in-the-blanks mode). However, there must be at least one language whose
statements are expressible, per some well-defined syntax, as character strings,
and that is comprehensive in supporting all of the following items:
- Data definition
- View definition
- Data manipulation (interactive and by program)
- Integrity constraints
- Transaction boundaries (begin, commit, and rollback)
- View updating rule — All views that are theoretically updateable are also updateable by the system.
- High-level insert, update, and delete — The capability of handling a base relation or a derived relation as a single operand applies not only to the retrieval of data, but also to the insertion, update, and deletion of data.
- Physical data independence — Application programs and terminal activities remain logically unimpaired whenever any changes are made in either storage representations or access methods.
- 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.
- 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.
- Distribution independence — A relational DBMS has distribution independence.
If a relational system has a low-level (single record at a time) language,
that low level cannot be used to subvert or bypass the integrity rules and
constraints expressed in the higher-level relational language (multiple records
at a time).