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In-Depth Information
terminology of Codd's paper entitled "A relational model of data for large shared data
banks," which was published in
Communications of the ACM
, Vol. 13, No. 6, June
1970, pp. 377-387. In this paper, Codd uses the terms
relation
,
attribute
, and
tuple
where more common programming usage refers to
table
,
column
, and
row
,
respectively.
The importance of Codd's ideas is such that the term "database" generally refers to a
relational database. Similarly, in common usage, a Database Management System,
or DBMS, generally means a Relational Database Management System. The terms
are used interchangeably in this chapter, and throughout the topic.
Codd's model covers the three primary requirements of a relational database:
structure, integrity, and data manipulation. The fundamentals of the relational model
are as follows:
A relational database consists of a number of unordered tables.
The structure of these tables is independent of the physical storage medium used to store the
data.
The contents of the tables can be manipulated using nonprocedural operations that return tables.
The implementation of Codd's relational model means that a user does not need to
understand the physical structure of the data in order to access and manage data in
the database. Rather than accessing data by referring to files or using pointers, the
user accesses data through a common tabular architecture. The relational model
maintains a clear distinction between the logical views of the data presented to the
user and the physical structure of the data stored in the system.
Codd based his model on a simple tabular structure, though his term for a table was a
relation
. Each table is made up of one or more rows (or
tuples
). Each row contains a
number of fields, corresponding to the columns or
attributes
of the table.
Throughout the rest of this topic, the more common programming terms are used:
table, column, and row. Generally, only database theorists use Codd's original
terminology; in that context, you are most likely to see references to relations,
attributes, and tuples.
The tabular structure Codd defines is simple and relatively easy for the user to
understand. It is also sufficiently general to be capable of representing most types of
data in virtually any kind of structure. An additional advantage of a tabular structure is
that tables are amenable to manipulation by a clearly defined set of mathematical
operations that generate results that are also in the form of tables. These
mathematical operations lend themselves readily to implementation in a high-level
language. In fact, Codd's rules require that a high level language be incorporated in
the RDBMS for just this purpose. That language has evolved into the Structured
Query Language, SQL, discussed in subsequent chapters.
