HIERARCHICAL DATABASE MODEL

A hierarchical database model is a data model in which the data are organized intoatree-like structure. Hierarchical model is one of the oldest database models, dating from 1960's. First Hierarchical database information management system was developed jointly by North American Rockwell company and IBM.

Hierarchy is an ordered tree and easy to understand. Data is represented in form of records. Relationship among the data is represented by records of links.Atree may be defined asaset of nodes. At the root of the tree is the single parent, the parent can have none, one or more children.

As it is arranged based on the hierarchy, every record of data tree should have at least one parent, except for the child records. The Data can be accessed by following through the classified structure, always initiated from the Root or the first parent. Hence this model is named as Hierarchical Database Model.

FEATURES OF HIERARCHICAL DATABASE MODEL:

• One to many relationships: It only supports one–to–many relationships. Many to many relationships are not supported.

• Problem in Deletion: Ifaparent is deleted then the child automatically gets deleted.

• Hierarchy of data: Data is represented inahierarchical tree-like structure.

• Parent-child relationship: Each child can have only one parent but a parent can have more than one children.

• Pointer: Pointers are used for linking records that tell which isa parent and which child record is.

• Disk input and output is minimized: Parent and child records are placed or stored close to each other on the storage device which minimizes the hard disk input and output.

• Fast navigation: As parent and child are stored close to each other so access time is reduced and navigation becomes faster.

• Predefined relationship: All relations between root, parent and child nodes are predefined in the database schema.

• Re-organization difficulty: Hierarchy prevents the re-organization of data.

• Redundancy: One to many relationships increases redundancy in the data which leads to the retrieval of inaCcurate data.

Hierarchical Model

Advantages

• Promotes data sharing
• Parent/child relationship promotes conceptual simplicity and data integrity
• Database security is provided and enforced by DBMS
• Efficient with 1:M relationships

Disadvantages

• Requires knowledge of physical data storage characteristics
• Navigational system requires knowledge of hierarchical path
• Changes in structure require changes in all application programs
• Implementation limitations
• No data definition
• Lack of standards