Welcome to the fifth module of our course on database design. In the last few lessons, we've built a solid conceptual foundation. We've defined entities, attributes, and relationships, and most recently, you learned how to precisely describe relationship constraints using cardinality and participation.
Now, it's time to translate those abstract concepts into a visual blueprint. This lesson is all about learning the graphical language of database designers: the standard symbols used in Entity-Relationship Diagrams (ERDs). Our goal is to identify the symbols for entities, attributes, and relationships. Mastering this visual vocabulary is the essential next step before you start drawing your own diagrams.
1. The Visual Language of ERDs
An ERD uses a set of standard symbols to represent the components of a database model. Think of it as a universal blueprint that any database designer can understand. The three main components we've discussed—entities, attributes, and relationships—each have a distinct graphical representation.
There are several notation styles for ERDs, but two are the most widespread:
- Chen Notation: An older, more conceptual style that uses distinct shapes for each component.
- Crow's Foot Notation: A more modern style, often preferred for implementation, that is more compact and visually represents relationship constraints clearly.
We will learn the symbols for both, as you may encounter either in your studies and in professional practice.
The image below provides a fantastic overview of the most common symbols. We will refer back to it as we explore each component in detail.
2. Chen Notation: The Conceptual View
Chen notation, developed by Peter Chen in 1976, is excellent for brainstorming and showing a high-level conceptual model. It uses specific shapes for each part of the model.
Let's begin with a reading that introduces the fundamental symbols of this notation.
This guide from Creately provides a clear breakdown of the most common ERD symbols, focusing primarily on the Chen notation style.
Please read the section titled Commonly Used ER Diagram Symbols and its subsections: Entity Symbols: Pay attention to the rectangle for a strong entity. Note the double rectangle for a weak entity, which we will discuss more in later lessons. Relationship Symbols: See how a diamond represents the relationship itself. Attribute Symbols: Notice the oval (ellipse) for attributes. Take note of the variations for different attribute types you learned about earlier, like the double ellipse for multivalued attributes. Connecting Lines: Understand that simple lines connect these shapes.
To see these symbols in action, the following short video provides a quick visual introduction.
The video "Entity Relationship Diagrams" visually demonstrates how Chen notation is used.
Watch the segment from where relationships are introduced to see how the diamond symbol connects entities. Then, watch from where attributes are added to see how ovals are connected to their parent entity.
In summary, the core of Chen notation is:
- Entity: A rectangle
- Relationship: A diamond
- Attribute: An oval
3. Crow's Foot Notation: The Implementation View
Crow's Foot notation is arguably more common in modern database design tools. It combines entities and their attributes into a single box, making the diagram more compact. Its real strength lies in the way it displays relationship constraints, which you are already familiar with from our last lesson.
The following reading explains this notation.
ERD alternatives and variations
This section from Runestone Academy explains the fundamentals of Crow's Foot notation.
Read the section 2.4.1.1. Entity-relationship modeling. Focus on how entities and attributes are represented in a single rectangle. Notice how relationships are just lines, with the symbols at the ends defining the cardinality and participation. This should be a good review from our previous lesson. The table provided is an excellent summary. Skip down to section 2.4.2.2. Attributes. Read how primary keys (PK) and foreign keys (FK) are indicated, usually with text styling or labels like "PK" and "FK".
As you can see, the main differences in Crow's Foot notation are:
- Entity & Attributes: A single rectangle with the entity name as the header and attributes listed below. There are no ovals.
- Relationship: A line connecting two entities. The name of the relationship is written on the line.
- Cardinality/Participation: The "crow's feet" and other symbols at the ends of the line represent the
(min, max)constraints, just as we learned.
4. Comparing the Notations
The best way to understand the difference is to see the same model drawn in both styles. While Chen notation is great for conceptual clarity, Crow's Foot is often more practical for detailed design because it's less cluttered and closer to how the final tables will be structured.
Here is a summary table comparing the primary symbols for each notation.
| Component | Chen Notation | Crow's Foot Notation |
|---|---|---|
| Entity | Rectangle | Rectangle with attributes listed inside |
| Attribute | Oval connected to an entity | Listed as text inside the entity's rectangle |
| Primary Key | Underlined attribute name in the oval | Marked with "PK" or underlined/bold text inside the box |
| Relationship | Diamond connecting two entities | A labeled line connecting two entities |
| Cardinality | Symbols (1, M, N) written next to the line | Specific symbols at the end of the line (e.g., ` |
The article from Runestone Academy that you read earlier also includes a side-by-side comparison diagram showing the same system modelled in both Chen and Crow's Foot, illustrating how much more compact the Crow's Foot version can be.
Conclusion
In this lesson, you learned to identify the standard symbols used in the two most common ERD notations: Chen and Crow's Foot. While their visual styles differ, they both represent the same fundamental database components.
Key Takeaways:
- Entities are represented by rectangles in both notations. In Crow's Foot, attributes are listed inside the rectangle.
- Attributes are shown as ovals in Chen notation but are simply text inside the entity box in Crow's Foot notation.
- Relationships are represented by a diamond in Chen notation and a simple line in Crow's Foot notation.
- Cardinality and participation are explicitly drawn using special symbols at the ends of the relationship line in Crow's Foot notation, making it very descriptive for implementation.
- The
ERD Symbols and Notationschart is a valuable reference for remembering these symbols.
Now that you can read the language of ERDs, you are ready to start writing it. In our next lesson, you will apply this knowledge to draw your first complete ERD for a simple business scenario.