Introduction to ER Diagram Notations
Entity-Relationship (ER) diagrams can be represented using different notation styles. Each notation style has its own set of symbols and conventions for depicting entities, relationships, and attributes. Understanding these notations is crucial for creating and interpreting ER diagrams effectively.
Several notation styles have evolved over time, each with its strengths and specific use cases. The choice of notation often depends on organizational standards, personal preference, and the specific tools being used.
Major ER Diagram Notation Styles
1. Chen Notation
Overview: Developed by Peter Chen in 1976, this is the original ER notation and is still widely used in academic settings.
Key Features:
- Entities represented as rectangles
- Attributes as ovals connected to entities
- Relationships as diamonds connecting entities
- Explicit cardinality notations (1, N, M) on relationship lines
Symbols:
- Entities: Rectangles with entity name inside
- Attributes: Ovals with attribute name inside
- Key Attributes: Underlined text in ovals
- Multi-valued Attributes: Double ovals
- Derived Attributes: Dashed ovals
- Relationships: Diamond shapes with relationship name inside
- Weak Entities: Double-lined rectangles
- Identifying Relationship: Double-lined diamond
Cardinality Representation:
- Written as “1”, “M”, “N” near the connection lines
- Often paired as 1:1, 1:N, M:N
Example: In a Chen diagram for a university database, “Student” and “Course” would be rectangles, “enrolls in” would be a diamond between them, and cardinality might show “N” near Student and “M” near Course to represent an M:N relationship.
2. Crow’s Foot Notation
Overview: Also known as Information Engineering (IE) notation, this is one of the most commonly used notations in industry.
Key Features:
- Simpler visual representation than Chen
- Entities as rectangles with attributes listed inside
- Relationships shown as lines
- Distinctive “crow’s foot” symbol to represent “many” cardinality
- No separate symbols for relationships
Symbols:
- Entities: Rectangles with entity name at top, attributes listed below
- Primary Key: Attributes marked with “PK”
- Foreign Key: Attributes marked with “FK”
- Mandatory Relationship: Solid line
- Optional Relationship: Dashed line
- One: Perpendicular line (|) at end of relationship line
- Many: Crow’s foot (three-pronged fork) at end of relationship line
- Zero or One: Circle and perpendicular line (o|)
- Zero or Many: Circle and crow’s foot (o<)
- One or Many: Perpendicular line and crow’s foot (|<)
Cardinality Representation:
- Combination of symbols at each end of relationship lines
- For example, |< represents “one to many”
Example: In a Crow’s Foot diagram, a line between “Department” and “Employee” entities would have a perpendicular line (|) at the Department end and a crow’s foot (<) at the Employee end, showing that one department has many employees.
3. IDEF1X Notation
Overview: Integration Definition for Information Modeling (IDEF1X) was developed by the U.S. Air Force and is used extensively in government and manufacturing.
Key Features:
- Focused on relational database design
- Strong emphasis on keys and normalization
- Distinguishes between identifying and non-identifying relationships
- Different box styles for different entity types
Symbols:
- Independent Entity: Rectangle with square corners
- Dependent Entity: Rectangle with rounded corners
- Key Attributes: Listed above a line within entity box
- Non-key Attributes: Listed below the line within entity box
- Identifying Relationship: Solid line
- Non-identifying Relationship: Dashed line
- Cardinality: Small circles (zero) or filled dots (one or more) on relationship lines
Cardinality Representation:
- Dot: One or more (required)
- Circle: Zero or more (optional)
- “P” label: Identifying parent in the relationship
Example: An “Order” entity (square corners) might connect to “OrderLine” entity (rounded corners) with a solid line, indicating OrderLine is dependent on Order with an identifying relationship.
4. UML Class Diagram Notation
Overview: Unified Modeling Language (UML) Class Diagrams can be adapted for ER modeling, especially in object-oriented environments.
Key Features:
- Similar to software engineering class diagrams
- Entities represented as classes
- Attributes and operations within class boxes
- Relationships shown with different line styles
- Includes multiplicity indicators
Symbols:
- Entities: Rectangles divided into three sections (name, attributes, operations)
- Attributes: Listed in middle section with data types
- Relationships: Various line styles connecting classes
- Association: Simple line
- Aggregation: Line with hollow diamond
- Composition: Line with filled diamond
- Generalization: Line with hollow triangle arrowhead
Cardinality Representation:
- Expressed as “multiplicity” notations (1, , 0..1, 1..) near the ends of relationship lines
Example: A “Student” class would connect to a “Course” class with a simple line, with “1..” near the Student end and “1..” near the Course end to show many-to-many relationship.
5. Bachman Notation
Overview: Developed by Charles Bachman, this notation focuses on representing the structure of network databases.
Key Features:
- Entities as boxes with rounded corners
- Relationships shown as lines with arrowheads
- Emphasis on owner-member relationships
Symbols:
- Entities: Boxes with rounded corners
- Relationships: Arrows pointing from owner to member
- One-to-many Relationship: Single arrow
- Many-to-many Relationship: Double arrows
Cardinality Representation:
- Direction of arrows indicates the “one” and “many” sides
Example: A “Department” entity would have an arrow pointing to an “Employee” entity, indicating a one-to-many relationship where Department is the owner and Employee is the member.
Comparative Analysis of ER Notations
Visual Clarity
- Chen: Very explicit but can become cluttered with many attributes
- Crow’s Foot: Cleaner representation, especially for complex diagrams
- IDEF1X: Clear distinction between entity types but more technical
- UML: Familiar to software developers but may be complex for non-technical users
- Bachman: Simple but limited in expressiveness
Expressiveness
- Chen: High - explicit representation of all components
- Crow’s Foot: Medium to high - can represent most concepts clearly
- IDEF1X: High for relational concepts, especially keys and dependencies
- UML: High - can represent complex relationships and constraints
- Bachman: Medium - focused on basic ownership relationships
Ease of Learning
- Chen: Medium - intuitive but many symbols to learn
- Crow’s Foot: High - relatively few symbols with intuitive meanings
- IDEF1X: Low to medium - requires understanding of relational concepts
- UML: Medium - familiar to many developers but has many variations
- Bachman: High - very simple notation
Tool Support
- Chen: Limited in modern tools, mostly academic
- Crow’s Foot: Widespread support in many database design tools
- IDEF1X: Supported in specialized tools, especially in government/defense sectors
- UML: Extensive support in software engineering tools
- Bachman: Limited support in modern tools
Choosing the Right Notation
The choice of ER notation depends on several factors:
1. Audience
- Non-technical Stakeholders: Simpler notations like Crow’s Foot might be more appropriate
- Database Professionals: More detailed notations like IDEF1X or Chen
- Software Developers: UML may be more familiar
2. Project Context
- Academic Setting: Chen notation is often preferred
- Business Applications: Crow’s Foot is commonly used
- Government Projects: IDEF1X may be required
- Object-Oriented Applications: UML is often most suitable
3. Tool Availability
- Select a notation supported by available modeling tools
- Consider compatibility with other diagrams in the project
4. Organizational Standards
- Many organizations standardize on a specific notation
- Consider existing documentation and team familiarity
Converting Between Notations
It’s sometimes necessary to convert between different notation styles:
Basic Conversion Guidelines
- Entities: Map directly between notations (rectangles in most notations)
- Attributes:
- From Chen: Move ovals into entity rectangles
- To Chen: Create ovals for each attribute
- Relationships:
- From Chen: Convert diamond relationships to lines with appropriate symbols
- To Chen: Convert relationship lines to diamonds with names
- Cardinality:
- Map specific symbols to their equivalent in the target notation
- Pay special attention to optional vs. mandatory participation
Common Pitfalls in Using ER Notations
- Mixing Notations: Accidentally combining elements from different notations
- Incorrect Cardinality Symbols: Using wrong symbols for the chosen notation
- Overcomplicating Diagrams: Including too much detail in a single diagram
- Inconsistent Use: Applying notation rules inconsistently across the diagram
- Tool Limitations: Using tools that don’t properly support the chosen notation
Best Practices for ER Diagram Notation
- Be Consistent: Use the same notation style throughout a project
- Document Your Notation: Provide a key for the symbols used
- Keep It Simple: Don’t overcrowd diagrams with unnecessary details
- Consider Your Audience: Choose notation appropriate for viewers’ expertise
- Validate Understanding: Ensure stakeholders correctly interpret the diagrams
- Use Layers or Views: Present different levels of detail for different audiences
- Follow Standards: Adhere to standard practices for your chosen notation
Understanding the various ER modeling notations allows database designers to effectively communicate database designs and choose the most appropriate representation for their specific context and audience.