Special Entity Types¶

ERD Guide — Part 3 of 6

Relationships with attributes, associative entities, and weak entities.

1. Relationships with Attributes¶

Sometimes a relationship itself has attributes that don't belong to either entity alone.

erDiagram
    EMPLOYEE }o--o{ VEHICLE : "reserves"

When an EMPLOYEE reserves a VEHICLE, the Date_Reserved doesn't belong to the employee or the vehicle — it belongs to the relationship between them.

How to show this: Place attributes on the relationship line in your ER diagram. In draw.io, these appear as labels on the connecting line.

2. Associative Entities¶

When a relationship has its own attributes, we often promote it to an associative entity — a full entity that represents the relationship.

Before (Relationship with Attributes)¶

erDiagram
    EMPLOYEE }o--o{ COURSE : "earns certificate"

The relationship carries: Certificate details, date earned, etc.

After (Associative Entity)¶

erDiagram
    EMPLOYEE ||--o{ CERTIFICATE : "earns"
    COURSE ||--o{ CERTIFICATE : "grants"

    CERTIFICATE {
        int CertificateID PK
        int EmployeeID FK
        int CourseID FK
        date DateEarned
        string Grade
    }

CERTIFICATE is now its own entity — it captures the relationship between EMPLOYEE and COURSE, plus has its own attributes.

When to Use Associative Entities

When a M:M relationship has attributes
When you need to track when or how two things are related
When the relationship itself could have relationships to other entities

3. Weak Entities¶

A weak entity cannot exist without its parent (or "strong") entity. It does not have a meaningful primary key on its own.

erDiagram
    EMPLOYEE ||--|{ DEPENDENT : "has"

    EMPLOYEE {
        int EmployeeID PK
        string Name
    }
    DEPENDENT {
        int EmployeeID FK
        string DependentName
        date DateOfBirth
        string Relationship
    }

A DEPENDENT (spouse, child) cannot exist in the database without being linked to an EMPLOYEE. If the employee record is deleted, the dependent records should also be removed.

Characteristics of Weak Entities¶

Property	Weak Entity	Regular Entity
Exists independently?	No — depends on parent	Yes
Has its own PK?	Partial key only (needs parent's PK)	Yes
Deletion behavior	Deleted when parent is deleted	Independent
Notation	Double-bordered rectangle (in some notations)	Single-bordered rectangle

Key Point

A weak entity's primary key is typically a composite key — the parent's primary key combined with a partial key that's unique only within that parent.

Example: DEPENDENT's PK = (EmployeeID, DependentName)

Summary¶

Concept	What It Is	When to Use
Relationship Attributes	Attributes that belong to the relationship, not either entity	When the relationship itself has data (dates, quantities, etc.)
Associative Entity	A relationship promoted to a full entity	M:M relationships with attributes, or when the relationship needs its own relationships
Weak Entity	An entity that can't exist without its parent	Child records that are meaningless without a parent (dependents, order line items, etc.)

← Degrees of Relationship | Next: Managing Attributes →