Siksha Sarovar

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Unit 1: ER Model - Entities and Notation

Lesson 6 of 28 in the free Database Management Systems notes on Siksha Sarovar, written by Rohit Jangra.

6.1 The Entity-Relationship (ER) Model

The ER Model is the most widely used conceptual data model for database design. It allows designers to represent the "world" as a set of objects (entities) and the associations among them (relationships).

6.2 Core Terminologies

1. Entity

An Entity is an object that exists in the real world and is distinguishable from other objects.

  • Concrete Entity: A specific person (Rahul), a specific car (Toyota Camry).
  • Conceptual Entity: A specific bank account, a specific course (DBMS).

2. Entity Type

A collection of entities that share the same attributes. In an ER diagram, this is represented by a Rectangle.

  • Example: The EMPLOYEE entity type represents all employees in a company.

3. Entity Set

The set of all entities of a particular entity type at any given point in time.

6.3 ER Diagram Notation (Standard)

ShapeMeaning
RectangleEntity Type
Double RectangleWeak Entity Type
EllipseAttribute
Underlined EllipseKey Attribute (Primary Key)
Double EllipseMultivalued Attribute
Dashed EllipseDerived Attribute
DiamondRelationship Type
Double DiamondIdentifying Relationship (for Weak Entities)
LinesConnects attributes to entities and entities to relationships

6.4 The Design Process

  1. Requirement Analysis: Understand what data the user needs.
  2. Conceptual Design: Create the ER diagram (High-level).
  3. Logical Design: Convert ER diagram to Tables (Relational Schema).
  4. Physical Design: Choose storage structures (Indexes, etc.).