Hierarchical model
Encyclopedia
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
.
'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 (see also nested set model
). Hierarchical data organization schemes resurfaced with the advent of XML
in the late 1990s http://www.ibm.com/developerworks/xml/library/x-matters8/index.html.
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:
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.
Data model
A data model in software engineering is an abstract model, that documents and organizes the business data for communication between team members and is used as a plan for developing applications, specifically how data is stored and accessed....
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.
One-to-many
One-to-many may refer to:* Multivalued function, a one-to-many function in mathematics* Fat link, a one-to-many link in hypertext* Point-to-multipoint communication, communication which has a one-to-many relation-See also:*One-to-one...
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
Information Management System
IBM Information Management System is a joint hierarchical database and information management system with extensive transaction processing capabilities.- History :...
developed by IBM and Windows Registry
Windows registry
The Windows Registry is a hierarchical database that stores configuration settings and options on Microsoft Windows operating systems. It contains settings for low-level operating system components as well as the applications running on the platform: the kernel, device drivers, services, SAM, user...
by Microsoft
Microsoft
Microsoft Corporation is an American public multinational corporation headquartered in Redmond, Washington, USA that develops, manufactures, licenses, and supports a wide range of products and services predominantly related to computing through its various product divisions...
.
History
The hierarchical data model lost traction as CoddEdgar F. Codd
Edgar Frank "Ted" Codd was an English computer scientist who, while working for IBM, invented the relational model for database management, the theoretical basis for relational databases...
's relational model
Relational model
The relational model for database management is a database model based on first-order predicate logic, first formulated and proposed in 1969 by Edgar F...
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 (see also nested set model
Nested set model
The nested set model is a particular technique for representing nested sets in relational databases.The term was apparently introduced by Joe Celko; others describe the same technique without naming it or using different terms...
). Hierarchical data organization schemes resurfaced with the advent of XML
XML
Extensible Markup Language is a set of rules for encoding documents in machine-readable form. It is defined in the XML 1.0 Specification produced by the W3C, and several other related specifications, all gratis open standards....
in the late 1990s http://www.ibm.com/developerworks/xml/library/x-matters8/index.html.
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:
|
|
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
Foreign key
In the context of relational databases, a foreign key is a referential constraint between two tables.A foreign key is a field in a relational table that matches a candidate key of another table...
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
Adjacency list
In graph theory, an adjacency list is the representation of all edges or arcs in a graph as a list.If the graph is undirected, every entry is a set of two nodes containing the two ends of the corresponding edge; if it is directed, every entry is a tuple of two nodes, one denoting the source node...
model, and was introduced by Dr. Edgar F. Codd
Edgar F. Codd
Edgar Frank "Ted" Codd was an English computer scientist who, while working for IBM, invented the relational model for database management, the theoretical basis for relational databases...
after initial criticisms surfaced that the relational model could not model hierarchical data.
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