Entity-Relationship Diagrams

An Entity-Relationship (E-R) Diagram is a visual representation of the entities in a database and the relationships between them. E-R diagrams are used during the design stage of database development to plan the structure before implementation.

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Core Components

1. Entities

An entity is a real-world object or concept about which data is stored. In an E-R diagram, entities are shown as rectangles.

Examples: Student, Course, Teacher, Order, Product.

2. Attributes

Attributes are the data items stored about each entity. They are shown as ovals connected to their entity (in Chen notation) or listed inside the entity box (in Crow's Foot notation).

Examples for a Student entity: StudentID, FirstName, Surname, DateOfBirth.

Key attribute: The primary key is usually underlined.

3. Relationships

Relationships show how entities are connected. They are shown as diamonds in Chen notation or as lines in Crow's Foot notation.

Examples: Student enrols on Course, Teacher teaches Course.

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Cardinality and Participation

Cardinality

Cardinality describes the number of instances of one entity that can be associated with instances of another.

Cardinality	Meaning	Notation
1:1	One-to-one	One instance of A relates to exactly one instance of B
1:M	One-to-many	One instance of A relates to many instances of B
M:M	Many-to-many	Many instances of A relate to many instances of B

Crow's Foot Notation

This is the most commonly used notation at A-Level. The symbols at each end of a relationship line indicate cardinality:

• A single line ( | ) means "one."
• A crow's foot ( ⊳ or fork shape ) means "many."
• A circle ( O ) means "zero" (optional participation).
• A bar ( | ) means "one" (mandatory participation).

Example: A Department has many Teachers.

Department ||------|< Teacher

Reading this: one Department has many Teachers. Each Teacher belongs to exactly one Department.

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Drawing E-R Diagrams — Step by Step

Step 1: Identify Entities

Read the scenario carefully and identify the main objects or concepts. Look for nouns:

"A library lends books to members. Each book has an author."

Entities: Library, Book, Member, Author.

Step 2: Identify Relationships

Look for verbs that connect entities:

• Library lends Book
• Member borrows Book
• Author writes Book

Step 3: Determine Cardinality

Ask: "How many of entity A relate to how many of entity B?"

• One Author writes many Books (1:M).
• One Member borrows many Books, and one Book can be borrowed by many Members over time (M:M).

Step 4: Identify Attributes