A Relational Model of Data for Large Shared Data Banks
Future users of large data banks must be protected from having to know how the 
data is organized in the machine (the internal representation).  A prompting
service which supplies such information is not a satisfactory
solution.  Activities of users at terminals and most application
programs should remain unaffected when the internal representation
of data is changed and even when some aspects of the external representation 
are changed.  Change in data representation will often be needed as a result 
of changes in query, update, and report traffic and natural growth in the 
types of stored information.  Existing noninferential, formatted data systems 
provide users with tree-structured files or slightly more general network 
models of the data.  In Section 1, inadequacies of these models are discussed.
A model based on n-ary relations, a normal form for data base relations,
and the concept of a universal form for data base relations,
and the concept of a universal data sublanguage are introduced.  In
Section 2, certain operations on relations (other than logical
inference) are discussed and applied to the problems
of redundancy and consistency in the user's model.
CACM June, 1970
Codd, E. F.
