Different Types of Database

There are three different types of data base. The difference lies in the organization of the database and the storage structure of the data. We shall briefly mention them here.

Relational DBMS 
This is our subject of study. A DBMS is relational if the data is organized into relations, that is, tables. In RDBMS, all data are stored in the well-known row-column format.

Hierarchical DBMS 
In HDBMS, data is organized in a tree like manner. There is a parent-child relationship among data items and the data model is very suitable for representing one-to-many relationship. To access the data items, some kind of tree-traversal techniques are used, such as preorder traversal.

Because HDBMS is built on the one-to-many model, we have to face a little bit of difficulty to organize a hierarchical database into row column format. For example, consider the following hierarchical database that shows four employees (E01, E02, E03, and E04) belonging to the same department D1.

  

There are two ways to represent the above one-to-many information into a relation that is built in one-to-one relationship. The first is called Replication, where the department id is replicated a number of times in the table like the following.

Dept-Id	Employee Code
D1	E01
D1	E02
D1	E03
D1	E04

Replication makes the same data item redundant and is an inefficient way to store data. A better way is to use a technique called the Virtual Record. While using this, the repeating data item is not used in the table. It is kept at a separate place. The table, instead of containing the repeating information, contains a pointer to that place where the data item is stored.

This organization saves a lot of space as data is not made redundant.