A music library, soccer system, or course catalog can be represented in several ways.
Two useful structural models are:
Both can show kinds of things, information about them, and relationships.
They are not the same diagram.
The easiest way to distinguish them is to ask:
What is the model trying to describe?
An ERD is primarily concerned with organized data.
For a soccer dataset, it might represent:
PLAYER
- Player ID
- Name
- Jersey Number
- Team ID
TEAM
- Team ID
- Team Name
with a relationship showing that teams and players are related.
The ERD helps describe:
The model supports questions about data organization.
A UML class diagram might represent:
Player
--------------------------------
playerId : string
name : string
jerseyNumber : int
Team
--------------------------------
teamId : string
name : string
with a relationship between Player and Team.
The class diagram helps describe:
The model supports questions about application structure.
An ERD may contain an entity named:
Player
A UML class diagram may contain a class named:
Player
That does not make the diagrams interchangeable.
The same real-world concept is being represented for two different purposes.
Both models may contain information such as:
Name
Jersey Number
An ERD may also emphasize record identifiers and reference fields such as:
Player ID
Team ID
A UML class diagram may emphasize software types such as:
name : string
jerseyNumber : int
The overlap exists because both models describe related information.
The differences exist because the models answer different questions.
In structured data, a relationship may be supported through matching identifiers.
For example:
Players.Team ID
↓
Teams.Team ID
The ERD represents that data relationship.
A UML class diagram may show:
Player -------- Team
to express that Player and Team objects are structurally related in the software model.
The exact implementation of that software relationship is not the main concern of this FIT activity.
How should the records and fields be organized and related?
What software types and attributes should represent this part of the system?
These questions can lead to similar-looking models without producing identical artifacts.
You can also compare both diagrams to the actual data table.
P001 | Jordan | 7 | T001
P002 | Casey | 1 | T001
This shows current records and values.
Player entity
Team entity
identifiers
attributes
relationship
This shows data structure.
Player class
Team class
typed attributes
software relationship
This shows software structure.
Three representations can describe the same general domain from different viewpoints.
Do not assume:
Every spreadsheet column becomes a UML class attribute.
Do not assume:
Every ERD entity becomes a class with exactly the same fields.
Do not assume:
Every class relationship should be implemented as an ID field.
Those conversions may sometimes be appropriate, but the model's purpose must guide the decision.
Choose an ERD when the main concern is:
Choose a UML class diagram when the main concern is:
Use the spreadsheet when the main concern is:
The strongest model is the one whose representation matches the question being answered.