Relationships are either mandatory or optional (must or May) Based on what you
Know about instances of the entities,
What is Cardinality in a Relationship?
Cardinality of a relationship determines the degree
of a relationship. It answers “how many.” (One or more than one)
Examples
Each EMPLOYEE must hold one and only one JOB
Each JOB may be held by one or more Employees
Each PRODUCT must be classified by one and only one PRODUCT TYPE
Each PRODUCT TYPE may classify one or more Products
Relationships
• Each SEAT may be sold to one or more PASSENGERS
• Each passenger may purchase one SEAT
• SEAT is sold to a PASSENGER(or PASSENGERS -- hence, overbooking
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