UML Class Diagram

UML (Unified Modeling Language) Class Diagrams are used for Conceptual Data Modeling — capturing the domain’s entities, attributes, and relationships at a high level before translating to a logical Entity-Relationship Diagram (ERD). UML is often preferred for conceptual modeling because it is more expressive and cleaner than ERD notation for representing type hierarchies and complex relationships.

Class Structure

A class has three compartments:

┌──────────────────┐
│     ClassName     │  ← Name
├──────────────────┤
│ - attribute1: Type│  ← Attributes
│ - attribute2: Type│
├──────────────────┤
│ + method(): Type  │  ← Operations (optional for DB modeling)
└──────────────────┘

Visibility markers: + public, - private, # protected, ~ package.

Relationship Types

Association

A domain-level connection between classes. Bidirectional by default.

Student ────── Course
        enrolls

Aggregation (◇──)

A whole-part relationship where parts can exist independently of the whole. Hollow diamond on the whole side.

Example: Department ◇── Employee (employees exist without the department)

Composition (◆──)

A stronger whole-part relationship where parts cannot exist independently. Filled diamond.

Example: Order ◆── OrderItem (order items don’t exist without the order)

Generalization (△──)

Class hierarchy / inheritance. Hollow triangle arrowhead.

Example: Person △── Student, Person △── Employee

Multiplicity

Annotations on relationship endpoints defining how many instances participate:

Notation	Meaning
1	Exactly one
0..1	Zero or one (optional)
* or 0..*	Zero or more
1..*	One or more (at least one)
n..m	Between n and m

Read from one class toward the other: “One instance of Class A is associated with [multiplicity] instances of Class B.”

Aggregation and Composition Patterns

The source material identifies several recurring patterns for recognizing aggregation and composition in a domain. Both are anti-symmetric and transitive (if A contains B and B contains C, then A contains C).

Pattern	Type	Example	Key Phrase
Assembly-Parts	Aggregation	Car ◇── Engine, Wheel	”is assembled from”
Container-Content	Aggregation	WebPage ◇── AdFrame	”holds”, “displays”
Organization-Member	Aggregation	Team ◇── Player	”consists of members”
Material-Object	Composition	Paper ◆── WoodFiber, Cotton	”is made from”
Portion-Ingredient	Composition	Pie ◆── Slice (homomeric)	“portion of”
Place-Area	Composition	Country ◆── State ◆── City	”is located in”
Collection-Members	Composition	WeeklyTimesheet ◆── DailyTimesheet	ordered, permanent parts

When in doubt, model the relationship as an association — it can always be refined into aggregation or composition when additional constraints are discovered.

Mapping to ERD

UML	ERD
Class	Entity
Attribute	Attribute
Association	Relationship with Foreign Key
Aggregation	Relationship (parts optional)
Composition	Relationship with CASCADE delete
Generalization	Collapsed via discriminator column, separate tables, or shared PK
Many-to-Many	Junction Table