Hierarchical database

Hierarchical model redirects here. For the statistics usage, see hierarchical linear modeling or hierarchical Bayesian model.

A hierarchical database model is a data model in which the data is organized into a tree-like structure. The structure allows representing information using parent/child relationships: each parent can have many children, but each child has only one parent (also known as a 1-to-many relationship). 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 each attribute as a column. Entity types are related to each other using 1:N mappings, also known as one-to-many relationships. This model is recognized as the first database model created by IBM in the 1960s.

Currently the most widely used hierarchical databases are IMS developed by IBM and Windows Registry by Microsoft.

Mother–child relationship: Child may only have one mother but a mother can have multiple children. Mothers and children are tied together by links called "pointers". A mother will have a list of pointers to each of her children.

History

The hierarchical structure was used in early mainframe DBMS. Records' relationships form a treelike model. This structure is simple but inflexible because the relationship is confined to a one-to-many relationship. The IBM Information Management System (IMS) and the RDM Mobile are examples of a hierarchical database system with multiple hierarchies over the same data. RDM Mobile is a newly designed embedded database for a mobile computer system.

The hierarchical data model lost traction as Codd's relational model became the de facto standard used by virtually all mainstream database management systems. A relational-database implementation of a hierarchical model was first discussed in published form in 1992[1] (see also nested set model). Hierarchical data organization schemes resurfaced with the advent of XML in the late 1990s[2] (see also XML database). The hierarchical structure is used primarily today for storing geographic information and file systems. Currently the most widely used hierarchical databases are IMS and Windows Registry by Microsoft.

Examples of hierarchical data represented as relational tables

An organization could store employee information in a table that contains attributes/columns such as employee number, first name, last name, and Department number. The organization provides each employee with computer hardware as needed, but computer equipment may only be used by the employee to which it is assigned. The organization could store the computer hardware information in a separate table that includes each part's serial number, type, and the employee that uses it. The tables might look like this:

EmpNo First Name Last Name Dept. Num
100 Sally Baker 10-L
101 Jack Douglas 10-L
102 Sarah Schultz 20-B
103 David Drachmeier 20-B
Serial Num Type User EmpNo
3009734-4 Computer 100
3-23-283742 Monitor 100
2-22-723423 Monitor 100
232342 Printer 100

In this model, the employee data table represents the "parent" part of the hierarchy, while the computer table represents the "child" part of the hierarchy. In contrast to tree structures usually found in computer software algorithms, in this model the children point to the parents. As shown, each employee may possess several pieces of computer equipment, but each individual piece of computer equipment may have only one employee owner.

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. The Windows Registry is a hierarchical database that stores configuration settings and options on Microsoft Windows operating systems.

See also

References

External links

  • Troels' links to Hierarchical data in RDBMSs
  • Managing Hierarchical Data in MySQL (This page is from archive.org as the page has been removed from MySQL.com)
  • Hierarchical data in MySQL: parents and children in one query
  • Create Hierarchy Chart from Hierarchical Database
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