Lesson 1 of 12

Structured learning draft

Entities with ERD

In Database Design & Modelling, the way a learner handles entities shapes how ERD is used and evaluated. An entity is a distinguishable thing represented consistently. This beginner lesson focuses on a decision or output that another person can inspect.

Learning objectives

Explain entities in the context of Database Design & Modelling.
Apply ERD to a bounded practical task.
Evaluate the result using explicit quality criteria.

Instructional figureNested structure

Entities: from context to evidence

Entities connects business rule and records to a valid data state in Database Design & Modelling.

Business rule and records

Define the purpose, intended user and ERD constraints.

frames

Entities

Derive entities from business rules and define stable identifiers.

produces evidence for

Valid data state

Compare the observed result with a normal case, boundary case and stated limitation.

Takeaway: Entities is credible only when the result can be traced back to its purpose, inputs and constraints. Entities is the decision layer between the starting context and evidence that the result is fit for purpose.

An entity is a distinguishable thing represented consistently. For ERD, distinguish performing an operation from demonstrating that it suits the stated purpose. Derive entities from business rules and define stable identifiers. Record assumptions that could change the conclusion.

Apply entities deliberately

State the Database Design & Modelling task and the decision it supports.
Prepare a small ERD case with a known input and difficult boundary.
Derive entities from business rules and define stable identifiers.
Compare the observed result with the expected behaviour and explain differences.
Save the evidence, limitation and next action in a review record.

entities evidence record
Review point	Evidence
Purpose	The specific ERD outcome and intended user
Method	The entities decision, input and version or context
Result	Observed output plus a checked boundary case
Limitation	What the result does not establish and the next safe action

Common mistakes

Using ERD before defining what entities must achieve.
Checking only the easiest Database Design & Modelling example.
Reporting a result without its input, assumptions or limitation.

Practice activity

Apply the lesson

For Database Design & Modelling, complete a bounded ERD task demonstrating entities. Keep the original input, numbered method, normal test, boundary test, observed results and a 100-word self-review naming one limitation and next improvement.

Check your understanding

In Database Design & Modelling, which evidence best supports a entities result produced with ERD?

The input, expected behaviour, normal and boundary results, and a limitation A polished ERD output without its input A screenshot showing the tool opened A claim that the method always works

Lesson summary

For Database Design & Modelling, entities means: An entity is a distinguishable thing represented consistently.
A credible ERD result includes a checked boundary, not only a successful example.
The next lesson builds on this entities evidence record.

Sources and further reading

PostgreSQL TutorialPostgreSQL Global Development Group - accessed 2026-08-21
SQL LanguagePostgreSQL Global Development Group - accessed 2026-08-21

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