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
What is an identifier attribute?
An attribute whose value distinguishes instances of an entity type
- StudentID
What is a relationship type?
A meaningful association between entity types
What is a relationship instance?
An entity that is connected to exactly one other entity
What is an associative entity?
An entity that is connected to many different entities
What is a mandatory relation?
Relation must occur once
- Each patient has a patient history
What is an optional relation?
Relation does not have to occur
- Each person can be married to another but doesn’t have to be
What is the degree of a relationship?
The number of entity types that participate in a relationship
What are the 3 different defined relationships we can have between entities?
Unary relationship
Binary relationship
Ternary relationship
What is a unary relationship?
A relationship between a single entity type
What is a binary relationship?
A relationship between two entity types
What is a ternary relationship?
A relationship between three entity types
What is a cardinality contraint?
A rule which specifies the number of relations an entity can have
What is a minimum cardinality?
The minimum number of relationships an entity can have
What is a maximum cardianlity?
The maximum number of relationships an entity can have
What are the two types of ER notation?
Crowfoot
CHEN notation
Describe a crowfoot diagram
- Entities are in boxes
- Attributes in the same box
- Relationships shown by crow feet
- Mandatory one ||
- Mandatory Many | crows
- Optional one o|
- Optional many o crows
Describe a chen notation diagram
- Entities are boxes
- Attributes are ovals connected
- Relationships are shown by lines with dots
- Mandatory one 1..1
- Mandatory many 1..M
- Optional one 0..1
- Optional Many 0..M
