chapter 3 Flashcards
what are the characteristics relationships in data modelling
- -> represent something of significance or importance to the business
- -> shows how entities are related to each other
- -> exist only between entities
- -> are bi-directional
- -> are named at both ends
- -> have optionality
- -> have cardinality
what is cardinality in a relationship
- -> cardinality measures the quantity of something
- -> in a relationship, it determines the degree to which one entity is related to another
what is an example of cardinalitu
each employee may hold one job and each job may be held by one or more employees
what is the rule that must be followed when writing erdish statements for cardinality
It is often necessary to add a small “s” to the end of the second entity name to make the sentence more readable.
Each JOB may be held by one or more EMPLOYEEs
ER diagramms are drawn according to the same _______-
conventiosn
how are entities represented on an ERD
they are represented as softboxes
the entity names are written inside the softboxes
what is ERDish
it is the language used to state relationships between entities in an ERD
what are the components of an erdish
EACH ENTITY A OPTIONALITY RELATIONSHIP NAME CARDINALITY ENTITY B
what is a matrix diagram
it is a grid-like drawing that can be used to discover and record relationships between entities in an entity-relationship model
it is a good way to make sure that you have not missed any relationships, especially when you are dealing with multiple entities
what does the matrix diagram not shwo
does not show optionality and cardinality
how many primary UID can an entity have
one
