Hi, this is Oracle Girl and today we are going to clarify what is a Relational Database and how the object structures are related. RDBMS or relational database management system is the most widely used database in the world. The logical structures are represented using ER (entity-relationship) model. An ER model has three characteristics – entity, relationship, attributes.
If we have “student” as an entity, then its attributes will be information that qualifies it. In our examples those attributes will be “name”, “grade” and “class”. We will create two more entities in order to demonstrate the remaining characteristic of the ER model – relationship.
There are three types of relationships:
“One to one relationship” is grade and subject – a student can have only one grade in certain subject at the end of the year.
“One to many relationship” – one student can be enrolled in many disciplines.
“Many to many relationship”- one discipline can have different teachers and teachers can have different disciplines. Many-to-many relationships should not exist in RDBMS because they cannot represent a single row properly.
In a relational database we need to be able to identify each occurrence of an entity uniquely. If we have two students or teachers with the same name we need a way to distinguish them. The easiest way is to add unique identifiers – like “stu_no”, “disc_no”, and “tea_no”.
When we take the above logical model and create database objects representing the entities and the relationships, we create the physical model – Relational database. In the physical model each entity becomes a table and its attributes become columns of that table. The defined relationship between the entities is part of one or more constraints between the tables. They are implemented to enforce data integrity.
Feel free to ask questions in the comments below if something is not clear enough.
Regards,
Oracle Girl
