In 1969 Dr. E. F. Codd laid down some 12 rules which a DBMS should adhere in order to get the logo of a true RDBMS.
Rule 1: Information Rule.
"All information in a relational data base is represented explicitly at the logical level and in exactly one way - by values in tables."
Rule 2: Guaranteed access Rule.
"Each and every datum (atomic value) in a relational data base is guaranteed to be logically accessible by resorting to a combination of table name, primary key value and column name."
In flat files we have to parse and know exact location of field values. But if a DBMS is truly RDBMS you can access the value by specifying the table name, field name, for instance Customers.Fields ['Customer Name'].
Rule 3: Systematic treatment of null values.
"Null values (distinct from the empty character string or a string of blank characters and distinct from zero or any other number) are supported in fully relational DBMS for representing missing information and inapplicable information in a systematic way, independent of data type.".
Rule 4: Dynamic on-line catalog based on the relational model.
"The data base 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."The Data Dictionary is held within the RDBMS, thus there is no-need for off-line volumes to tell you the structure of the database.
Rule 5: Comprehensive data sub-language 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 the following items
Data Manipulation (Interactive and by program).
Transaction boundaries ( Begin , commit and rollback)
Rule 6: .View updating Rule
"All views that are theoretically updatable are also updatable by the system."
Rule 7: 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."
Rule 8: Physical data independence.
"Application programs and terminal activities remain logically unimpaired whenever any changes are made in either storage representations or access methods."
Rule 9: Logical data independence.
"Application programs and terminal activities remain logically unimpaired when information-preserving changes of any kind that theoretically permit un-impairment are made to the base tables."
Rule 10: Integrity independence.
"Integrity constraints specific to a particular relational data base must be definable in the relational data sub-language and storable in the catalog, not in the application programs." Rule 11: Distribution independence.
"A relational DBMS has distribution independence."
Rule 12: Non-subversion Rule.
"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)."