Modelling Databases: Part 1
To model a database, we have to first understand the business requirements at conceptual level, which is later translated into a relational database.
For understanding the business requirements at a conceptual level, we use
Core Concepts in ER Model
Entity
Real world objects/concepts are called entities in ER Model.
Attributes of an Entity
Properties of real world objects/concepts are represented as attributes of an entity in ER model.
Key Attribute
The attribute that uniquely identifies each entity is called key attribute.
Entity Type
Entity Type is a collection of entities that have the same attributes (not values).
Relationships
Association among the entities is called a relationship.
Example:
- Person has a passport.
- Person can have many cars.
- Each student can register for many courses, and a course can have many students.
Types of relationships
- One-to-One Relationship
- One-to-Many or Many-to-One Relationship
- Many-to-Many Relationship
One-to-One Relationship
An entity is related to only one entity, and vice versa.
Example
- A person can have only one passport.
- similarly, a passport belongs to one and only one person.
One-to-Many Relationship
An entity is related to many other entities.
Example
- A person can have many cars. But a car belongs to only one person.
Many-to-Many Relationship
Multiple entities are related to multiple entities.
Example
- Each student can register to multiple courses.
- similarly each course is taken by multiple students.
Cardinality Ratio
Cardinality in DBMS defines the maximum number of times an instance in one entity can relate to instances of another entity.
