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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
Relationship constructs

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.
crowsfoot construct
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. 1:1 relationship
  2. 1:N relationship
  3. Unresolved M:N relationship
  4. 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.
Crowsfoot Elements ER Diagram