Entity relationship modelling Flashcards
What does an ER diagram aim to do?
Facilitate database design
What is an entity?
A person, place, object, event or concept
What is an entity type?
A collection of entities that share common properties or characteristics
What is an entity instance?
A single occurrence of an entity type
What types of entity can we have?
- Strong entity type
- Weak entity type
Explain a strong entity type
An entity type that exists independently of other entity types
Explain a weak entity type
An entity type who’s existence depends on some other entity type
Give a practical example of a strong and weak entity type
Person is a strong entity type, license is a weak entity type (t relies on the existence of people)
What is the identifying owner of an entity type?
The entity type on which the weak entity type depends on
What is the identifying relationship of an entity type?
The relationship between a weak entity type and its owner
What is an attribute in terms of entitys?
A property or characteristic of an entity or relationship type
What are the different types of attribute we can have?
- Simple (atomic) attribute
- Composite attribute
- Multivalued attribute
- Identifier
What is a simple (atomic) attribute?
An attribute that cannot be broken down into smaller components
- Name
What is a composite attribute?
An attribute that has meaningful component parts
- Address
What is a multivalued attribute?
An attribute that may take on more than one value for a given entity
- Skill
