ER Diagram Symbols

An Entity-Relationship (ER) Diagram is a visual representation of entities, attributes, and their relationships in a database. It uses standardized symbols to depict different components, which makes it easier to design and understand the database structure before implementation.

1. Entity

• Symbol: Rectangle
• Meaning: Represents a real-world object or concept.

Types:

Strong Entity: Normal rectangle.
Example: Student

Weak Entity: Double rectangle.
Example: Dependent (depends on Employee)

2. Attribute

• Symbol: Ellipse (Oval)
• Meaning: Denotes a property or characteristic of an entity or relationship.

Types:

Simple (Atomic): Single oval
Example: Name

Composite: Oval with smaller ovals branching
Example: FullName → FirstName, LastName

Multivalued: Double oval
Example: PhoneNumbers

Derived: Dashed oval
Example: Age (derived from DateOfBirth)

Key Attribute: Underlined text in the oval
Example: RollNo (Primary Key)

3. Relationship

• Symbol: Diamond
• Meaning: Shows how two or more entities are associated.

Types:

Regular Relationship: Normal diamond
Example: Enrolled between Student and Course

Identifying Relationship: Double diamond (used with weak entities)

4. Lines (Connections)

• Straight Line: Connects attributes to entities and entities to relationships.
• Double Line: Used to connect a weak entity to its identifying relationship.

5. Cardinality & Participation

Cardinality (Multiplicity): Indicates how many entities participate in a relationship.

• 1:1 → One-to-one
• 1:N → One-to-many
• M:N → Many-to-many

Symbols:

• Often shown as numbers (1, N, M) or notations like (1,1), (0,N), etc.

Participation:

• Total Participation: Double line (entity must participate)
• Partial Participation: Single line (entity may or may not participate)

Quick Symbol Reference Table

By using these ER diagram symbols, students can effectively design and analyze database schemas, ensuring clarity in structure and data relationships. It’s a foundational skill for DBMS design and normalization.