Database Reference
In-Depth Information
Oneadvantagethenetworkdatabaseprovidesisfastdataaccess.Italsoallowsuserstocre-
ate queries that are more complex than those they created using a hierarchical database. A
network database's main disadvantage is that a user has to be very familiar with the struc-
ture of the database in order to work through the set structures. Consider the Agents data-
basein
Figure1.3
onceagain.Itisincumbentontheusertobefamiliarwiththeappropriate
set structures if she is to determine whether a particular engagement has been paid. Anoth-
er disadvantage is that it is not easy to change the database structure without affecting the
application programs that interact with it. Recall that a relationship is explicitly defined as
a set structure in a network database. You cannot change a set structure without affecting
the application programs that use this structure to navigate through the data. If you change
a set structure, you must also modify all references made from within the application pro-
gram to that structure.
Although the network database was clearly a step up from the hierarchical database, a few
people in the database community believed that there must be a better way to manage and
maintain large amounts of data. As each data model emerged, users found that they could
askmorecomplexquestions,therebyincreasingthedemandsmadeuponthedatabase.And
so we come to the relational database model.
The Relational Database Model
The relational database was first conceived in 1969 and is still one of the most widely used
database models in database management today. The father of the relational model, Dr.
Edgar F. Codd, was an IBM research scientist in the late 1960s and was at that time look-
ing into new ways to handle large amounts of data. His dissatisfaction with the database
models and database products of the time led him to begin thinking of ways to apply the
disciplines and structures of mathematics to solve the myriad problems he had been en-
countering. Being a mathematician by profession, he strongly believed that he could apply
specificbranchesofmathematicstosolveproblemssuchasdataredundancy,weakdatain-
tegrity, and a database structure's over dependence on its physical implementation.
Dr.Coddformallypresentedhisnewrelationalmodelinalandmarkworkentitled“ARela-
el on two branches of mathematics—set theory and first-order predicate logic. Indeed, the
name of the model itself is derived from the term
relation,
which is part of set theory. (A
widely held misconception is that the relational model derives its name from the fact that
tables within a relational database can be related to one another.)
A relational database stores data in relations, which the user perceives as tables. Each re-
lation is composed of tuples, or records, and attributes, or fields. (I'll use the terms
tables,
records,
and
fields
throughouttheremainderof the topic.)Thephysicalorderoftherecords
