Conceptual data models and entity relationship modelling revision notes

Purpose

A conceptual data model is a structured representation of the data requirements for a scenario. It identifies the different entities involved, the attributes that describe them, and the relationships between the entities. The model should reflect the requirements rather than adding unrelated data.

Identifying entities and attributes

Read the scenario carefully and look for the important things that need to be represented. These are possible entities, such as Student, Course or Booking. For each entity, identify the attributes that describe it. For example, a student may have a student number and a name, while a course may have a course code and a title. An attribute should describe the entity it is listed under.

Entity identifiers

Each entity needs an attribute, or set of attributes, that forms its entity identifier. This identifies an entity occurrence within the model. Underlining can be used to show the identifier in an entity description. For example:

Student (StudentNumber, Name, DateOfBirth)

If StudentNumber is the identifier, it can be represented as:

Student (__StudentNumber__, Name, DateOfBirth)

The same notation can be used for other entities, such as Course (__CourseCode__, Title).

Relationships and diagrams

An entity relationship diagram represents the entities and the relationships between them. The diagram should match the written requirements. If the scenario says that students enrol on courses, the model should show a relationship between Student and Course. Do not create a relationship merely because two entities appear in the same scenario; it should be supported by the requirements.

Common errors

Common mistakes include treating an attribute as an entity, omitting an important entity, failing to identify the entity identifier, or drawing a relationship that is not supported by the requirements. Check that every attribute is attached to the correct entity and that the diagram and entity descriptions agree.