Several logical data structures are used to
express the relationships between individual data elements or records in a
database. Common logical data structures are hierarchical, network, and
relational, with relational being predominant.
In a hierarchical structure sometimes
referred to as a tree structure, the stored data get more and more detailed as
on branches further and further out on the tree. Each segment, or node, may be
subdivided into two or more subordinate nodes, which can be further subdivided
into two or more additional nodes. However, each node can emanate form only one
"parent". To the user, each record resembles an organizational chart
in which the segments or nodes fit into a well-defined hierarchy or tree. There
is only one segment, or root, at the top.
Applications programs process hierarchical
databases one record at a time, as with conventional file structures.
Hierarchical database structures are commonly used with large mainframe computer
The network structure is similar to the
hierarchical structure with the exception that in the network structure, a node
may have more than one parent. The trade-off between the simplicity of design of
a hierarchical structure and the storage efficiency of a network structure is a
very important consideration in database implementation. Network structures are
most commonly used with mainframe and minicomputer systems, rarely with
The relational structure organizes data in
two-dimensional tables. These tables offer great flexibility and a high degree
of data security. The relational structure uses relatively little memory or
secondary storage. Unfortunately, the process of creating these tables is rather
elaborate. Another disadvantage of this structure is that it generally requires
more time to access information than does either of the other two structures.
This is because much more information must be searched in order to answer
queries posed to the system. In addition, some implementations use a fixed
amount of storage for each field, resulting in inefficient storage utilization.
In spite of these disadvantages, the relational structure has gained rapid
acceptance and is currently the most popular of the three structures. This
structure is used almost exclusively with microcomputer systems and is
increasingly being applied to minicomputer and mainframe systems. Many experts
predict that it will eventually replace the other structures completely.