The Hierarchical Data Model is a way of organizing a database with multiple one to many relationships. The structure is based on the rule that one parent can have many children but children are allowed only one parent. This structure allows information to be repeated through the parent child relations created by IBM and was implemented mainly in their Information Management System. (IMF), the precursor to the DBMS.
The model allows easy addition and deletion of new information. Data at the top of the Hierarchy is very fast to access. It was very easy to work with the model because it worked well with linear type data storage such as tapes. The model relates very well to natural hierarchies such as assembly plants and employee organization in corporations. It relates well to anything that works through a one to many relationship. For example; there is a president with many managers below them, and those managers have many employees below them, but each employee has only one manager.
This model has many issues that hold it back now that we require more sophisticated relationships. It requires data to be repetitively stored in many different entities. The database can be very slow when searching for information on the lower entities. We no longer use linear data storage mediums such as tapes so that advantage is null. Searching for data requires the DBMS to run through the entire model from top to bottom until the required information is found, making queries very slow. Can only model one to many relationships, many to many relationships are not supported. Clever manipulation of the model are required to make many to may relationships. For example; what if a professor teaches classes, and is also a graduate student?
Example of Hierarchical Data model