Entity Relationship Model - Relations

Question 1

Cardinality

Answer 1

Is the min and max number of entity occurrences associated with one occurrence of the related entities

• E.g. you may not want more than 30 students in 1 class
• The cardinality can be expressed as (1,N) when there is no upper limit on the number of instance relations

Question 2

Existence-Dependent meaning

Answer 2

when an entity can only exist when the associated entity exists

(in implementation terms: the FK cannot be empty)

Question 3

Existence-Independent meaning

Answer 3

when an entity can exist apart from all related entities

Question 4

Weak (non-identifying) Relationship Strength

Answer 4

when the PK of an entity contains a primary key component of the parent entity

Furthermore - it must be existence-dependent, it cannot exist without the entity in which it has the relationship

Question 5

Strong (identifying) Relationship Strength

Answer 5

when the PK of an entity does not contain the primary key of a parent element

Question 6

Relationship participation

Answer 6

Relationships can be mandatory or optional - if they are optional, then one entity does not require an associated related entity

This will be reflected in the Cardinality of the relationship (if it’s optional, it’ll be (0, x)