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Logical Data Structures
 

   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.

 

Hierarchical Structure

   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 systems.

 

Network Structure

   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 microcomputers.

 

Relational Structure

   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.



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