1. Relationships—Topics
Relationship Cardinality & Notation
Policy and Cardinality

2. Identify “Information Elements” Identify Entities
Database Design Steps
Identify “Information Elements”
Identify Entities
Identify Primary Keys
Identify Relationships
Fix Relationship Problems
Identify Non-Key Attributes
Fix Attribute Problems
Address Performance Issues
Build Application Objects (stored procedures, views)
Design Reports
List is not strictly sequential
Some steps performed recursively
Some steps require returning to previous steps

3. Cardinality
Understanding “Cardinality” is one of the most fundamentally important concepts in DB design
Cardinality indicates how many occurrences of an entity must or may be allowed in the relationship with any one occurrence in the other entity
Cardinality goes in each direction
One student may/must take ? Classes
One class must/may be taken by ? Students

4. Relationship Cardinality (cont.)
The measure of cardinality has two components at each end of the relationship:
A maximum (usually either 1 or an unconstrained number greater than one, referred to as “many”)
A minimum (usually either 0 or 1 but other values are possible, though rare)
Relationship is mandatory if at least one matching record is required (minimum is 1)
Relationship is optional if a matching record is not required (minimum is 0)

5. Cardinality Notation Entity Entity Entity Entity Mandatory One
One professor must have exactly one phone number
Mandatory Many
A customer must have at least one purchase to be a customer but may have many
Optional One
One professor may have as few as zero reserved parking spaces but may have only one at most
Optional Many
One student may take as few as zero classes but may take more than one class
Entity
Entity
Entity

6. Cardinality Notation (cont.)
Interpret these cardinalities

7. Cardinality Notation (cont.)
Relationship cardinality is governed by the number of related occurrences you could have
If a student could have two majors then relationship is ‘Many’ on the Major side
May a car or house have more than one owner?
May an Employee be assigned to more than one job title at a time?
Will you record a Supplier if you do not currently carry any of their products?
Will you enter an Employee without assigning them to a position?

8. Cardinality Notation (cont.)
Two Common Beginner Problems
Historical Data:
Moment of Creation Test:

9. Cardinality Notation (cont.)
Commonly used verbal shorthand ignores the minimum component of a relationship
1:M (one-to-many)
1:1 (one-to-one)
M:M (or M:N) (many-to-many)

10. Cardinality Notation (cont.)
The graphical layout of a relationship is purely arbitrary

11. Organization Policy and Cardinality
Business policies (or regulations) may affect cardinality

12. Exercise
Refer back to your entity diagram you created in Module 1 (Exercise #3)
Identify the cardinalities at both ends of each relationship in the exercise
Minimum cardinality
Maximum cardinality