One of the earliest database management systems was based on the hierarchical model. In a hierarchical data model, the records have a parent-child relationship.
Consider the application used for Production Planning in automobile manufacturing companies. The model of the database is shown in the following figure. The automobile manufacturer manufactures various models of cars. Each car model is decomposed into its assemblies (Engine, Body and Chassis). Each assembly is further decomposed into sub-assemblies (valves, spark plugs…), and so on. If the manufacturer wants to generate the Bill of Materials for a particular model of an automobile, the hierarchic data model would be suitable because the bill of materials for a product has a hierarchical structure. Each record represents a particular part and since the records have a parent-child relationship each part is linked to its subpart.
The Hierarchical Model
Characteristics of Hierarchical Model:
The characteristics of a hierarchical DBMS are as follows:
1. Representation of Data as Hierarchical Trees:
The hierarchical database is characterized by parent-child relationships between records. A record type R1 is said to be the parent of record type R2 if R1 is one level higher than R2 in the hierarchical tree. The root of the hierarchy is the most important record type and all records at different levels of the hierarchy are dependent on the root.
2. Each Sub-Module has only one Super-Module:
Each child record (sub-module) has only one parent record (super-module). The parent record can have one or more children record types.
3. Represents a set of Related Records:
There can be one or more than one record occurrence for a given record type. When one writes into the database, only one occurrence of the record type is written. Similarly, whenever a record is retrieved from the database only one occurrence of the record type is retrieved.
4. Hierarchy through Pointers:
In a hierarchical database, pointers link the records. Pointers determine whether a particular record occurrence is a parent or a child record. The path from the parent to the child is maintained through pointers.
5. Simple Structure:
The database is a simple hierarchical tree. The parent and child records are stored close to each other on the disk, minimizing disk input and output.
6. High Performance:
The parent-child relationship is stored as pointers from one record to another. So navigation through the database is very fast resulting in high performance.
7. Pre-defined Relationship between Record Types:
Record types at different levels of the hierarchy are dependent on the root. Root is the most important record type in the hierarchy.
8. Difficult to Reorganize:
It is difficult to reorganize the database because the hierarchy has to be maintained. Each time a record type is inserted or deleted the pointers have to be manipulated to maintain the parent-child relationship.
9. More Complex Real life Requirements:
The hierarchical DBMS is based on a simple parent-child relationship, but real life applications are more complex and cannot be represented by a hierarchical structure. For example, in an order-processing database, a single order might participate in three different parent-child relationships linking the order to the customer who placed the order, the items ordered and the sales person who took the order. This complex structure cannot be represented in a hierarchical structure.
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