Entities, attributes, identifiers
The building blocks of a data model: the entity and its occurrences, attributes (simple, composite, calculated), the identifier and the data dictionary. Relationships come in the next deck.
- 25 cards
- 5 topics
- Free
The outline, topic by topic
Each topic is weighted the way the exam weighs it.
- Concepts
- Questions
- Sort and link
- Scenarios
- Understand
A few cards from the deck
Entity: what is it?
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A category of objects or people in the domain being studied about which information must be kept, all described by the same properties.
STUDENT and VEHICLE are entities of a driving school. Its name is singular: the entity is the type, not the collection.
A library keeps for each member: surname, first name, date of birth, card number. Which attribute should be the identifier of MEMBER?
- The card number
- The surname
- The surname and first name together
- The date of birth
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The card number
Only the card number is given to one person and one person only. Two members can share a surname, or even a surname and first name, or a date of birth.
An entity groups … that are all described by the same ….
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An entity groups occurrences that are all described by the same attributes.
If two sets of objects do not have the same attributes (students and vehicles), they are two distinct entities.
Occurrence of an entity: what is it?
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One particular member of the category, described by its own values: Ms Durand among the students, or the car registered AB-123-CD among the vehicles.
The entity describes the shape, the occurrences are the real cases: in the future table, the entity becomes the table and each occurrence a row.
A garage only wants to record each vehicle's colour, with nothing else about colours. How should it be modelled?
- As a COLOUR entity, because a colour is an object of the real world
- As an occurrence of VEHICLE
- As a colour attribute of VEHICLE
- As the identifier of VEHICLE
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As a colour attribute of VEHICLE
A colour has no property of its own here: it is a plain value that describes the vehicle. It would become an entity if more had to be kept (code, paint supplier).