Entity Relationship Diagram Symbols and Meanings Explained

If you've ever stared at an ER diagram and felt lost because the shapes and lines looked like a foreign language, you're not alone. Entity relationship diagram symbols and meanings form the visual vocabulary of database design. Knowing them well means you can read, build, and communicate database structures clearly whether you're designing a new system or explaining one to a team. Get them wrong, and you end up with miscommunication, flawed schemas, and hours of rework.

What are entity relationship diagram symbols?

An entity relationship diagram (ERD) uses specific shapes and connectors to represent how data is organized in a database. Each symbol carries a precise meaning. The main symbols you'll encounter are entities (rectangles), attributes (ovals or columns), relationships (diamonds or lines with labels), and cardinality indicators (symbols at the ends of lines that show how many instances relate to each other).

These symbols aren't decorative. A rectangle with rounded corners means something different from a sharp-cornered rectangle in certain notations. A crow's foot at the end of a line tells you "many," while a single line or circle tells you "one" or "zero." Every shape and mark encodes a specific fact about your database structure.

Why do different ER diagram notations use different symbols?

This is one of the most common points of confusion. There isn't just one way to draw an ER diagram. The three most widely used notations are Chen notation, Crow's Foot notation (also called Martin notation), and UML class diagrams. Each uses different symbols for the same concepts.

For example, in Chen notation, a relationship is shown as a diamond shape connecting two entities. In Crow's Foot notation, there's no diamond relationships are represented by lines with specific endpoint markers. UML uses a different visual style altogether, resembling class diagrams with labeled associations.

Understanding these differences matters when you're reading diagrams created by other people or using different tools. A comparison of Crow's Foot and Chen notation can help you figure out which style you're looking at and what each symbol means within that context.

What do the main ER diagram symbols mean?

Here's a breakdown of the core symbols and what they represent, starting with the most common notation styles.

Entity symbols

An entity represents a real-world object or concept that stores data like "Customer," "Order," or "Product."

Chen notation: Entities are drawn as rectangles.
Crow's Foot notation: Entities are also rectangles, often shown as a box with the entity name in a header row and attributes listed below it.
Weak entity: In Chen notation, a weak entity (one that depends on another entity for its identification) is drawn with a double-bordered rectangle.

Attribute symbols

Attributes describe the properties of an entity. For a "Customer" entity, attributes might include name, email, and phone number.

Chen notation: Attributes are ovals connected to their entity by a line.
Crow's Foot notation: Attributes are listed as rows inside the entity box. A primary key is typically marked with an icon like a key symbol or "PK" label.
Derived attribute: Shown with a dashed oval in Chen notation this means the value is calculated from other attributes (like "age" derived from "date of birth").
Multivalued attribute: Shown with a double oval in Chen notation meaning an entity can have multiple values for that attribute (like multiple phone numbers).
Composite attribute: An oval connected to smaller ovals, representing an attribute that can be broken into sub-parts (like "full name" split into "first name" and "last name").

Relationship symbols

A relationship describes how two entities interact with each other.

Chen notation: Relationships are diamonds containing a verb (like "places" between Customer and Order).
Crow's Foot notation: Relationships are lines between entities, with a label placed on or near the line. There's no diamond shape.
Identifying relationship: Drawn as a solid line in many notations, this connects a strong entity to a weak entity where the weak entity's identity depends on the strong one.
Non-identifying relationship: Drawn as a dashed line in some notations, indicating the related entity has its own independent primary key.

Cardinality and participation symbols

Cardinality tells you how many instances of one entity can be associated with instances of another. This is where many people get tripped up, because the symbols differ across notations.

Crow's Foot notation:
- A single line (or "one") at the end of a relationship line means exactly one.
- A crow's foot (three-pronged fork) means "many."
- A circle (o) means "zero."
- A vertical line (|) or the line itself means "one."
- Common combinations: "one-to-many" (|──< on one side), "zero-or-many" (o──<), "one-and-only-one" (||), "zero-or-one" (o|).
Chen notation: Cardinality is indicated with numbers (1, N, M) placed on the connecting lines near the entities. You might see "1:N" to mean one-to-many, or "M:N" for many-to-many.

If cardinality symbols trip you up, reading a dedicated walkthrough on how to read ER diagram cardinality can clear things up with visual examples.

What's the difference between strong and weak entity symbols?

A strong entity has its own primary key and exists independently. A weak entity depends on another entity for identification it can't be uniquely identified on its own.

In Chen notation:

Strong entity = single-bordered rectangle
Weak entity = double-bordered rectangle
Weak relationship (identifying relationship) = double-bordered diamond
Partial key (discriminator) of a weak entity = underlined attribute in a dashed oval

In Crow's Foot notation, weak entities are sometimes marked with a notation key in the diagram legend, or the relationship line is styled differently (dashed vs. solid) depending on the tool you're using.

A practical example: an "Order" entity might be strong, while an "OrderLineItem" entity is weak because it only makes sense in the context of a specific order.

What do the different relationship types look like on an ER diagram?

There are three fundamental relationship types, and each one looks slightly different depending on the notation:

One-to-One (1:1): Each instance of Entity A relates to exactly one instance of Entity B. In Crow's Foot, both ends show single lines or "||" symbols. Example: one User has one Profile.
One-to-Many (1:N): One instance of Entity A relates to many instances of Entity B. In Crow's Foot, the "one" side shows a single line, the "many" side shows a crow's foot. Example: one Department has many Employees.
Many-to-Many (M:N): Many instances of Entity A relate to many instances of Entity B. In Crow's Foot, both ends show crow's feet. Example: Students enroll in many Courses, and Courses have many Students.

In physical database design, many-to-many relationships are usually resolved with a junction table (also called an associative entity or bridge table). The ER diagram might show this as an intermediate entity with two one-to-many relationships.

How do you read total vs. partial participation in ER diagrams?

Total participation means every instance of an entity must be involved in the relationship. Partial participation means some instances might not participate.

In Chen notation: Total participation is shown with a double line connecting the entity to the relationship diamond. Partial participation uses a single line.
In Crow's Foot notation: A minimum cardinality of "one" (represented by a straight line or "|") signals total participation. A minimum of "zero" (shown as a circle "o") signals partial participation.

Example: If every Order must have a Customer (total participation on the Order side), you'd draw a double line in Chen notation. But if a Customer might exist in the system without having placed any orders, that's partial participation on the Customer side a single line.

What common mistakes do people make with ER diagram symbols?

Several pitfalls come up regularly when working with these symbols:

Mixing notations: Using Chen-style diamonds with Crow's Foot-style cardinality markers creates confusion. Pick one notation and stay consistent throughout your diagram.
Confusing cardinality with participation: Cardinality is about "how many" (one or many). Participation is about "minimum" (zero or one). They're related but not the same thing.
Skipping the legend: If others will read your diagram, always include a notation key or legend that explains which symbols you're using.
Overcomplicating with too many attributes: Including every single column in your ER diagram clutters it. Focus on key attributes that define the entity and its relationships.
Forgetting about normalization: A diagram that looks correct symbolically might still have structural problems. Following best practices for normalization ensures your relationships and entities are properly structured before you move to implementation.
Ignoring relationship labels: A line between "Customer" and "Order" without a label leaves readers guessing. Always label relationships with a verb that describes the interaction.

Which notation should you use for your ER diagram?

It depends on your audience and your tool:

Use Chen notation if you're in an academic setting, teaching database concepts, or working with a formal data model. The explicit diamonds and ovals make concepts very visible.
Use Crow's Foot notation if you're working in a professional development environment. It's compact, widely understood, and supported by most database design tools (MySQL Workbench, Lucidchart, draw.io, Microsoft Visio, dbdiagram.io).
Use UML notation if your team already works with UML for software design and you want your ER diagrams to fit that visual language.

There's no universally "right" choice. What matters is that everyone reading the diagram agrees on which notation is being used.

Quick-reference checklist for ER diagram symbols

✅ Entity: Rectangle (single border for strong, double border for weak in Chen notation)
✅ Attribute: Oval (Chen) or listed row inside the entity box (Crow's Foot)
✅ Primary Key: Underlined attribute (Chen) or PK label (Crow's Foot)
✅ Relationship: Diamond with verb label (Chen) or labeled line (Crow's Foot)
✅ Cardinality "one": Single line or "||" marker
✅ Cardinality "many": Crow's foot symbol (three-pronged fork)
✅ Participation "zero": Circle (o) at the line endpoint
✅ Total participation: Double line (Chen) or minimum cardinality of one
✅ Derived attribute: Dashed oval (Chen)
✅ Multi-valued attribute: Double oval (Chen)

Before you start your next diagram, pick your notation, set up a legend, and sketch the entities and relationships first. Add attributes and cardinality details once the structure feels right. If you want to deepen your understanding, this overview of ER diagram basics from Lucidchart covers the concepts with interactive visual examples.