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Data models
Main articles:Data modelandDatabase model
A data model is an abstract structure that provides the means to
effectively describe specific data structures needed
to model an application. As such a data model needs sufficient
expressive power to capture the needed aspects of
applications. These applications are often typical to commercial
companies and other organizations (like
manufacturing, human-resources, stock, banking, etc.). For
effective utilization and handling it is desired that a datamodel
is relatively simple and intuitive. This may be in conflict with
high expressive power needed to deal with certain
complex applications. Thus any popular general-purpose data
model usually well balances between being intuitive
and relatively simple, and very complex with high expressive
power. The application's semantics is usually not
explicitly expressed in the model, but rather implicit (and
detailed by documentation external to the model) and hinted
to by data item types' names (e.g., "part-number") and their
connections (as expressed by generic data structure
types provided by each specific model).
[edit]Early data models
These models were popular in the 1960s, 1970s, but nowadays can
be found primarily in oldlegacy systems. They
are characterized primarily by beingnavigationalwith strong
connections between their logical and physical
representations, and deficiencies indata independence.
[edit]Hierarchical model
Main article:Hierarchical database model
In the Hierarchical model different record types (representing
real-world entities) are embedded in a predefined
hierarchical (tree-like) structure. This hierarchy is used as
the physical order of records in storage. Record access is
done by navigating through the data structure
usingpointerscombined with sequential accessing.
This model has been supported primarily by the IBMIMSDBMS, one
of the earliest DBMSs. Various limitations of
the model have been compensated at later IMS versions by
additional logical hierarchies imposed on the base
physical hierarchy.
[edit]Network model
Main article:Network model (database)
In this model a hierarchical relationship between two record
types (representing real-world entities) is established via
the setconstruct. A set consists of circularlinked listswhere
one record type, the set owner or parent, appears once
in each circle, and a second record type, the subordinate or
child, may appear multiple times in each circle. In this
way a hierarchy may be established between any two record types,
e.g., type A is the owner of B. At the same time
another set may be defined where B is the owner of A. Thus all
the sets comprise a generaldirected
graph(ownership defines a direction), ornetworkconstruct. Access
to records is either sequential (usually in each
record type) or by navigation in the circular linked lists.
This model is more general and powerful than the hierarchical,
and has been the most popular before being replaced
by the Relational model. It has beenstandardizedbyCODASYL.
Popular DBMS products that utilized it wereCincom
Systems' Total andCullinet'sIDMS.
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