ER Diagram Cardinality Symbols

In this tutorial, we will learn about different ER diagram Cardinality Symbols. The relationships are indicated using lines drawn from one table to another and a symbol at the end that defines the relationship type. The ERD acronym stands for Entity-Relationship Diagram. An ER Diagram is an output of the modeling process.

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Cardinality

Cardinality is the maximum number of times an instance of an entity can be associated with instances of another entity, as shown in the ER diagrams. Ordinality is the minimum number of times an instance of an entity can be associated with an instance in another entity.

Cardinality Symbols

Cardinality/Ordinality are shown by styling the relationship line and its endpoint with cardinality symbols. Some basic symbols in Crow’s Foot notation are as follows:

 

 

Examples

1-to-0 ( or 1) Relationship

1-to-0 ( or 1) Relationship can be represented as shown below. Each instance of Entity A is related to a minimum of zero and a maximum of one instance of Entity B.

 

Customers – Orders – Shipments

Let’s look at another simple example. A customer can visit an e-commerce website and place orders to buy things.  The website sells goods and services to its customers. The e-commerce website ships the items to the customer.

 

A customer can place multiple orders, which can contain multiple shipments or none. For example, if the customer buys a service like recharging his/her mobile, then there is no shipment for the order.

 

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