Hierarchical database model
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- Hierarchical model redirects here. For the statistics usage, see hierarchical linear modeling.
A hierarchical data model is a data model in which the data is organized into a tree-like structure. The structure allows repeating information using parent/child relationships: each parent can have many children but each child only has one parent. 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 a column. Entity types are related to each other using 1: N mapping, also known as one-to-many relationships.
The most recognized and used hierarchical databases are IMS developed by IBM and Windows Registry by Microsoft.
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[edit] History
A relational database implementation of this type of data model was first discussed in publication form in 1992[1] (see also nested set model).
[edit] Example
An example of a hierarchical data model would be if an organization had 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 date of birth. The Employee table represents a parent segment and the Children table represents a Child segment. These two segments form a hierarchy where an employee may have many children, but each child may only have one parent.
Consider the following structure:
| EmpNo | Designation | ReportsTo |
|---|---|---|
| 10 | Director | |
| 20 | Senior Manager | 10 |
| 30 | Typist | 20 |
| 40 | Programmer | 20 |
In this, the "child" is the same type as the "parent". The hierarchy stating EmpNo 10 is boss of 20, and 30 and 40 each report to 20 is represented by the "ReportsTo" column. In Relational database terms, the ReportsTo column is a foreign key referencing the EmpNo column. If the "child" data type were different, it would be in a different table, but there would still be a foreign key referencing the EmpNo column of the employees table.
This simple model is commonly known as the adjacency list model, and was introduced by Dr. Edgar F. Codd after initial criticisms surfaced that the relational model could not model hierarchical data.
[edit] See also
[edit] References
- ^ Michael J. Kamfonas/Recursive Hierarchies: The Relational Taboo!—The Relation Journal, October/November 1992
[edit] External links
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