Database Design
3-1: Identifying Relationships
Practice Activities
Objectives
• I nterpret and describe relationship optionality
• I nterpret and describe relationship cardinality
• R elate entities by applying the rules of cardinality and optionality
Vocabulary
Identify the vocabulary word for each definition below.
Cardinality A property of an end of a relationship between X and Y that
describes how many of X is related to Y.
relationship A connection or association between objects.
Optionality A property of an end of a relationship between X and Y that
describes whether X must be or may be related to Y.
Try It / Solve It
1. Read the given business scenario. Name the relationships between EMPLOYEE and
JOB. Include appropriate optionality and cardinality.
“We have a lot of employees who handle one or more different jobs. We’d like to keep
track of who is working on which job. Although employees can help each other, a job is
assigned to one employee and is the ultimate responsibility of that employee. All of our
employees have at least one job. However, jobs exist that are not yet assigned to
anyone.”
Cardinality and Optionality : Employee must have one or more job
2. Read the given business scenario. Name the relationships between ORDER and
WAITER. Include the appropriate optionality and cardinality.
“We assign our waiters to certain areas, except for our trainees who just observe and are not
responsible for taking any orders yet. A waiter takes the orders for the tables in his area. All
areas have one assigned waiter. A customer places an order with a waiter. If the customer has
a question or wants to make a change to the order, he needs to request this with the assigned
waiter.”
, Optionality: Assign waiters to certain areas except for our trainees who observe.
Customer may change order with the waiter
Cardinality: All areas have one assigned waiter.
3. Read the given business scenario. Name the relationships between PARENT and CHILD.
Include the appropriate optionality and cardinality.
“At the end of each day, parents need to pick up their children at our day-care center. All
children must be picked up by 6 p.m. A child may have two parents, but we need only one of
them to come and pick up the child. We cannot release a child to anyone but that child’s
parent.”
Optionality: Parents need to pick up children at the day-care center
Cardinality: Children must be picked up by 6. Child needs one parent to pick them up
4. Read the given business scenario. Name the relationships between TEACHER and
STUDENT. Include the appropriate optionality and cardinality.
“Some students request remedial help in certain subjects, such as math. We can assign a tutor
who can work with the student after class. Some of our teachers agree to be tutors. If several
students need tutoring in one subject, then we assign them to the same teach-
er. If a student needs tutoring in several subjects, then he will probably be assigned to
several different tutors.”
Optionality: Students may request a tutor. Teachers may agree to be tutors as well as
students.
5. Draw two entities that you are familiar with from your own experience (use colored pens
and markers on hand). Write down the relationships between these two entities, including
cardinality and optionality. Share your entities with the class.
3-1: Identifying Relationships
Practice Activities
Objectives
• I nterpret and describe relationship optionality
• I nterpret and describe relationship cardinality
• R elate entities by applying the rules of cardinality and optionality
Vocabulary
Identify the vocabulary word for each definition below.
Cardinality A property of an end of a relationship between X and Y that
describes how many of X is related to Y.
relationship A connection or association between objects.
Optionality A property of an end of a relationship between X and Y that
describes whether X must be or may be related to Y.
Try It / Solve It
1. Read the given business scenario. Name the relationships between EMPLOYEE and
JOB. Include appropriate optionality and cardinality.
“We have a lot of employees who handle one or more different jobs. We’d like to keep
track of who is working on which job. Although employees can help each other, a job is
assigned to one employee and is the ultimate responsibility of that employee. All of our
employees have at least one job. However, jobs exist that are not yet assigned to
anyone.”
Cardinality and Optionality : Employee must have one or more job
2. Read the given business scenario. Name the relationships between ORDER and
WAITER. Include the appropriate optionality and cardinality.
“We assign our waiters to certain areas, except for our trainees who just observe and are not
responsible for taking any orders yet. A waiter takes the orders for the tables in his area. All
areas have one assigned waiter. A customer places an order with a waiter. If the customer has
a question or wants to make a change to the order, he needs to request this with the assigned
waiter.”
, Optionality: Assign waiters to certain areas except for our trainees who observe.
Customer may change order with the waiter
Cardinality: All areas have one assigned waiter.
3. Read the given business scenario. Name the relationships between PARENT and CHILD.
Include the appropriate optionality and cardinality.
“At the end of each day, parents need to pick up their children at our day-care center. All
children must be picked up by 6 p.m. A child may have two parents, but we need only one of
them to come and pick up the child. We cannot release a child to anyone but that child’s
parent.”
Optionality: Parents need to pick up children at the day-care center
Cardinality: Children must be picked up by 6. Child needs one parent to pick them up
4. Read the given business scenario. Name the relationships between TEACHER and
STUDENT. Include the appropriate optionality and cardinality.
“Some students request remedial help in certain subjects, such as math. We can assign a tutor
who can work with the student after class. Some of our teachers agree to be tutors. If several
students need tutoring in one subject, then we assign them to the same teach-
er. If a student needs tutoring in several subjects, then he will probably be assigned to
several different tutors.”
Optionality: Students may request a tutor. Teachers may agree to be tutors as well as
students.
5. Draw two entities that you are familiar with from your own experience (use colored pens
and markers on hand). Write down the relationships between these two entities, including
cardinality and optionality. Share your entities with the class.
