Hierarchical model

From Free net encyclopedia

In a hierarchical data model, data is organized into a tree-like structure in such a way that it cannot have too many relationships. The structure allows repeating information using parent/child relationships. All attributes of a specific record are listed under an entity type. In a database, an entity type is the equivalent of a table; each individual record is represented as a row and an attribute as columns. Entity types are related to each other using 1: N mapping also known as one to many relationships. An example would be: an organisation has records of employees in a table (entity type) called Employees. In the table there would be attributes/columns such as First Name, Last Name, Job Name and Wage. The company also has data about the employee’s children in a separate table called Children with attributes such as First Name, Last Name, and DOB. The Employee table represents a parent segment and the Children table represents a Child segment. These 2 segments form a hierarchy where an employee may have many children but each child may only have 1 parent.

Hierarchical structures were widely used in the first mainframe database management systems. However, owing to their restrictions, they often cannot be used to relate structures that exist in the real world. Hierarchical relationships between different types of data can make it very easy to answer some questions, but very difficult to answer others. If a one-to-many relationship is violated (e.g., a patient can have more than one physician) then the hierarchy becomes a network.

1 Terms
2 Some well known hierarchical databases
3 See also
4 External links

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Terms

Field – smallest unit of data
Segment – groups of fields; nodes of the tree structure
Database record – a collection of related segments; a particular tree structure
Database – composed of database records
Database description – how database records are defined; set of assembly-language macro instructions
Root – first segment
Sequence field – one field in each segment used to order the occurrences of a given type

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