Lesson 5	Diagramming types of relationships
Objective	Identify the constructs for diagramming types of relationships.

Diagramming Relationship Types

Like all ER diagrams, the Crowsfoot version provides constructs that represent each type of entity relationship.These constructs are illustrated below:

Relationship constructs — The diagram provided visually represents cardinality in entity-relationship (ER) modeling using crow's foot notation, a standard in relational database design. Here's an analysis of each relationship type shown:

One-to-One Relationship Notation:

A straight line with a single vertical bar (|) at both ends.

Meaning:

Each entity in Table A is related to at most one entity in Table B, and vice versa.

Example: Each person has one passport, and each passport belongs to one person.

One-to-Many Relationship Notation:

One end has a single vertical bar (|), and the other end has a "crow's foot" (three-pronged symbol).

Meaning:

An entity in Table A can be associated with many entities in Table B, but each entity in Table B is related to only one in Table A.

Example: One customer can place many orders, but each order is placed by only one customer.

Many-to-Many Relationship Notation:

Both ends of the line have "crow's foot" symbols.

Meaning:

Multiple entities in Table A can relate to multiple entities in Table B.

Example: A student can enroll in many courses, and each course can have many students.

**Summary Table:**

Relationship Type Symbol Description

One-to-One ` --------- ` Unique pairing on both sides

One-to-Many ` --------<` One on left, many on right

Many-to-Many `>--------<` Many on both sides

These relationships are essential in defining the logic between tables in relational databases, enabling correct schema design and query formulation.

Relationship Type	Symbol	Description
One-to-One	`	---------	`	Unique pairing on both sides
One-to-Many	`	--------<`	One on left, many on right
Many-to-Many	`>--------<`	Many on both sides

Three-pronged "Crowsfoot" Symbol

Notice that the three-pronged "crowsfoot" symbol is simply a miniaturization of the "many" side of a relationship that we discussed in an earlier module. The "one" side of the relationship is indicated by a short vertical line. The graphic below illustrates a one-to-many relationship and its corresponding Crowsfoot construct.

The following series of images illustrates the Crowsfoot constructs for

1:1,
1:N, and
unresolved M:N relationships, as well as the same M:N relationship resolved:

Four Relationships described with Crow's Foot Notation

1) 1:1 relationship — **1:1 relationship:** 1. Each employee is assigned to an office and an office is assigned to only one employee

2) 1:N relationship — **1:N relationship:** 2. Each Product originates from a single supplier but each supplier can furnish many products

3) Unresolved M:N relationship — **Unresolved M:N relationship:** 3. Each class consists of many students and each student can take many classes

4) Resolved M:N relationship — **Resolved M:N relationship:** 4. This is how you illustrate the resolved version of the same M:N relationship

1:1 relationship
1:N relationship
Unresolved M:N relationship
Resolved M:N relationship

Crows Foot Notation
The symbols used to indicate the types of participation (mandatory and optional) are placed directly on the lines comprising the relationship constructs, as you will see in the next lesson.

ER diagram

Before moving on to the next lesson, click the link below to check your understanding of the elements of an ER diagram.

Elements of Crow's foot ER Diagram

Some of the graphic elements of an ER diagram with the things they are used to indicate. Here are the elements and their correct matches:

ER diagram and correct matches — This image is a visual reference for Crow’s Foot Notation, a popular modeling convention used in Entity-Relationship Diagrams (ERDs). Let’s analyze each of the components shown:
🔷 Entity

**Symbol**: A plain rectangle divided into two parts.

**Explanation**: Represents a real-world object or concept in the database, typically mapped to a table.

The top part usually holds the entity name.

The bottom part holds attributes (not shown in this specific diagram).

🔗 1\:N Relationship (One-to-Many)

**Symbol**: A line with a single vertical bar on one end and a crow’s foot on the other.

**Explanation**: Indicates that one instance of the first entity can be related to many instances of the second entity.

Example: One customer places many orders.

🔗 1:1 Relationship (One-to-One)

**Symbol**: A line with a single vertical bar on both ends.

**Explanation**: Indicates a strict one-to-one mapping between two entities.

Example: Each person has one unique passport.

🔗 M\:N Relationship (Many-to-Many)

**Symbol**: A line with a crow’s foot on both ends.

**Explanation**: Each instance of both entities can relate to multiple instances of the other.

Example: Students enroll in many courses; each course has many students.

🧩 Composite Entity (Associative Entity)

**Symbol**: A rectangle with **cut-off corners** (typically a composite/bridge table).

**Explanation**: Used to resolve a many-to-many relationship into two one-to-many relationships.

This entity includes foreign keys referencing the related entities and can hold **attributes unique to the relationship**.

Example: `Enrollment` as a composite entity between `Student` and `Course`.

**Summary Table**

Notation Meaning Example

Entity Table or object `Customer`, `Product`

1:N Relationship One record links to many others `Customer` ↔ `Order`

1:1 Relationship One record links to one other `Person` ↔ `Passport`

M:N Relationship Many records link to many others `Student` ↔ `Course`

Composite Entity Resolves M:N into 1:N + 1:N `Enrollment` between two tables

This notation is crucial for accurately visualizing and designing relational databases.

Notation	Meaning	Example
Entity	Table or object	`Customer`, `Product`
1:N Relationship	One record links to many others	`Customer` ↔ `Order`
1:1 Relationship	One record links to one other	`Person` ↔ `Passport`
M:N Relationship	Many records link to many others	`Student` ↔ `Course`
Composite Entity	Resolves M:N into 1:N + 1:N	`Enrollment` between two tables

If you have any questions about the constructs used in ER diagrams in general, or about this exercise in particular execute in the Search box to uncover more information from this website.