Hierarchical Model

            A hierarchical model is one of the earliest database management systems.  The hierarchical model is a record based data model.  Record based model is used to describe external and conceptual level of a database and describe some extent of an internal level of a database.  In record based data model database consist of records and these records may be of different type. In hierarchical model records are stored as a parent child relationship. Information management system (IMS) is a most popular hierarchical database system.  Information Management System is introduced by IBM in 1968 and still used by IBM mainframes.

The example of a hierarchical model:

                                              Figure: The hierarchical model

Characteristics of hierarchical model:

                                                A hierarchical model has different characteristics.  The characteristics of a hierarchical database management system are as follow:

1.      In hierarchical database records are stored as a parent child relationship. For example record R1 is called a parent of a record type R2. The root of a hierarchical is very important and all records at different level of a hierarchy are dependent on the root.  The record that is called a parent is one level higher than a record that is called a child of that parent.

2.       Each child record has only one parent record, but the parent record has one or many children record types. Parent record is also called super module and child record is called sub module.

3.      The record type has one or more record occurrences. When written into database than only one occurrence of a record type is written, and when a record is retrieve from database than only one occurrence of a record type is retrieve.

4.        Particular record occurrence is a parent or child record is determined by Pointer. The records in hierarchical database are link to each other trough pointer. The path from parent to child is maintained through pointer.

5.       The structure of a hierarchical database is simple. The parent and child records are stored close to each other on the disc it minimize the disc input and output.

6.    In hierarchical database records are stored as a pointer from one record to another record. Due to pointer, to move from one place to another place is very fast resulting in high performance.

7.     Root is very important record in the hierarchy. Record type at different level of a hierarchy is dependent on root.

8.     It is very difficult to reorganize database. To maintained

 Parent-Child relationship pointer is manipulated each time a record is inserted or deleted in the database.

9.    Hierarchical database is based on Parent-Child relationship. The application of a real life is more complex that way it cannot be represented by hierarchical structure.