Cardinality in Entity-Relationship Diagrams¶

ERD Guide — Part 1 of 6

Understanding how entities relate to each other through cardinality.

What is Cardinality?¶

Cardinality describes how one instance of an entity relates to instances of another entity. It answers the question:

"One instance of Entity A relates to how many instances of Entity B?"

We explore the cardinality between entities in both directions, one direction at a time:

From A → B: "One instance of A relates to ___ instance(s) of B"
From B → A: "One instance of B relates to ___ instance(s) of A"

Maximum Cardinality: The Three Basic Types¶

1. One-to-One (1:1)¶

One instance of the first entity relates to exactly one instance of the second entity.

erDiagram
    EMPLOYEE ||--|| VEHICLE : "is assigned"

Example: One employee is assigned one company vehicle. One vehicle is assigned to one employee.

2. One-to-Many (1:M)¶

One instance of the first entity relates to many instances of the second entity.

erDiagram
    CUSTOMER ||--o{ ORDER : "places"

Example: One customer can place many orders. But each order belongs to only one customer.

3. Many-to-Many (M:M)¶

Many instances of the first entity relate to many instances of the second entity.

erDiagram
    CUSTOMER }o--o{ PRODUCT : "purchases"

Example: One customer can purchase many products. One product can be purchased by many customers.

Resolving Many-to-Many Relationships¶

Important

Many-to-Many relationships are typically resolved by creating a new table that captures the matches between the two original tables.

The M:M relationship between CUSTOMER and PRODUCT becomes:

erDiagram
    CUSTOMER ||--o{ ORDER : "places"
    PRODUCT ||--o{ ORDER : "appears in"

    ORDER {
        int OrderID PK
        int CustomerID FK
        int ProductID FK
    }

The new ORDER table sits between CUSTOMER and PRODUCT, turning one M:M into two 1:M relationships.

Minimum Cardinality: Optional vs. Mandatory¶

Beyond maximum cardinality (one or many), we also specify whether a relationship is required or optional.

Optional Relationships (Minimum = 0)¶

An instance of one entity may or may not be related to an instance of the other.

Optional One-to-One (0..1)¶

erDiagram
    CUSTOMER |o--o| ORDER : "may have"

A customer may or may not have an order.

Optional One-to-Many (0..M)¶

erDiagram
    CUSTOMER |o--o{ PRODUCT : "may purchase"

A customer may or may not have purchased any products. (Zero or many.)

Mandatory Relationships (Minimum = 1)¶

An instance of one entity must be related to at least one instance of the other.

Mandatory One-to-One (1..1)¶

erDiagram
    CUSTOMER ||--|| ORDER : "must have"

Every customer must have at least one order. (Perhaps they're only added to the system when they place an order.)

Mandatory One-to-Many (1..M)¶

erDiagram
    CUSTOMER ||--|{ PRODUCT : "has purchased"

Every customer must have purchased at least one product.

Notation Summary¶

Symbol	Meaning
`\|\|`	Exactly one (mandatory)
`\|o`	Zero or one (optional)
`\|{`	One or many (mandatory)
`o{`	Zero or many (optional)

How to Determine Cardinality¶

Follow this process for any two entities:

Pick a direction — start with Entity A → Entity B
Ask: "Can ONE instance of A relate to ONE or MANY instances of B?"
Ask: "Is the relationship MANDATORY or OPTIONAL?"
Repeat in the other direction (B → A)
Combine both sides to get the full cardinality notation

Practice Tip

Always think about specific instances, not the entities in general. Ask: "Can this ONE specific customer have many orders?" not "Can customers have orders?"

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