Codd's Rules

Rule 1 : The 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."

Everything within the database exists in tables and is accessed via table access routines.

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."