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Introduction to RDBMS : Day 1 : Chapter 1

In this chapter, you will learn to:

What is Object-Based Logical Model ?

Object-based logical model,

1. Describes the data at the conceptual and view levels.
2. Provide fairly enough structuring capabilities.
3. Allow one to specify data constraints explicitly.
4. Object based Model provides more than 30 models, including
   • Entity-relationship model.
   • Object-oriented model.
   • Binary model.
   • Functional data model.

There are various object-based models. The most widely used is the Entity-Relationship model (ER model) introduced by Peter Chen.  So Now we are going to focus on this model.

Example 1  : A sample Entity-Relationship model, where
Entities: SUPPLIER, PARTS
Relationship: SHIP (or SHIPMENT)

Example 2 : A detailed Entity-Relationship model

Entities: CUSTOMER,ACCOUNT
Relationship: DEPOSIT
Attributes of Customer (Entity): Customer ID, Name,Street,City
Attributes of Account (Entity): Account Number and Balance

What are Entities in Entity-Relationship model ?

• Chen defined an entity as “a thing, which can be easily identified”.
• An entity is any object, place, person, or activity about which data is recorded.
• In the diagramming technique, entities are named and represented inside a box.
• An entity type is a set of things that share common properties:
  • STUDENT, COURSE, and GRADE are examples of entity type.
  • An entity type is usually in uppercase.

What are the Types of Entities ?

1. Dependent entity: Is an entity whose existence depends on the existence of another entity and are also called weak entities
2. Independent entity: Is an entity which does not depend on any other entity for existence and are also called regular entities

Define Relationships in Entity-Relationship model ?

Chen defines a relationship as “an association among entities”. For example, the relationship between students and instructors  represents the fact that an instructor teaches several students and a student is taught by several instructors. This relationship could be named TEACH.

Relationships are depicted as a diamond with the name of the relationship type.

What are Types of Relationships are available in Entity-Relationship (ER) model?

1. One-to-One
2. One-to-Many (or Many-to-One)
3. Many-to-Many

Examples:

One-to-One

Consider the example of a university. For one DEPARTMENT (like the department of social sciences) there can be only one department head. This is a one-to-one relationship.

One-to-Many (or Many-to-One)

A STUDENT can MAJOR in only one course, but many STUDENTs would have registered for a given MAJOR course. This is a many-to-one relationship.

Many-to-Many

A STUDENT can take many COURSEs and many STUDENTs can register for a given COURSE. This is a many-to-many relationship

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