The Power of ONE
Today, we will evaluate chapter 5 of this
book and continue to teach you the
essential database concepts, technology,
and techniques that you will need to
begin a career as a database developer.
5.1 Explain how entities are transformed into
relations.
To transform an E-R data model into a relational database design,
a relation is created for each entity. The attributes of the entity
become the columns of the relation, and the identifier of the entity
becomes the primary key of the relation.
5.2 Why is it necessary to apply the normalization process to the
relations created according to your answer in question 5.1?
It is necessary to apply the normalization
process to the relations created because the
process is then applied to each relation, and
additional relations are created. The
attributes of the functional dependencies
out of the relation, making a copy of their
determinants in the original relation as
foreign keys.
5.3 What is denormalization?
Denormalizationthe process of intentionally designing a relation that
is not normalized. Denormalization is done to improve
performance or security.
5.4 When is Denormalization justified?
Denormalization is justified when breaking groups into
separate tables will make the designs hard to use.
5.5 explain the problems that unnormalized relations have for insert,
update, and delete actions.
Insert: If we leave Zip, City, and State in CUSTOMER, then we will
not be able to insert data for a new zip code until a customer has that
zip code. However, we will never want to do that. We only care about
zip code data when one of our customers has that zip code. Therefore,
leaving the zip data in CUSTOMER does not pose problems when
inserting.
Update: If a city changes its zip code, then we might have to change
multiple rows in CUSTOMER. How frequently do cities change their
zip code though? Because the answer is almost never, updates in the
denormalized relation are not a problem.
Delete: If only one customer has the zip data (80210, Denver,
Colorado), then if we delete that customer, we will lose the fact
that 80210 is in Denver. This does not really matter though because
when another customer with this zip code is inserted, that customer
also will provide the city and state.
5.6 Explain how the representation of weak entities differs from the representation of
strong entities.
Weak entities logically depend on another entity. Strong entities are not
dependent on another table since they have an “ID” dependency.
5.7 List the three types of binary relationships and give an example of each.
The simplest form of binary relationship is a one-to-one (1:1) relationship, in which an entity of one type is related to at most one
entity of another type.
Ex: Employee to Cubicle Number. Each single employee is assigned to their own single cubicle
The second type of binary relationship, known as one to many (1:N), is a relationship in which an entity of one type can be related
to many entities of another type.
Ex: Counselor to Student. One high school counselor is assigned to many students.
The third and final type of binary relationship is many to many (M:N), in which an entity of one type corresponds to many entities
of the second type, and an entity of the second type corresponds to many entities of the first type.
Ex: A student too many classes and a class to many students. Many-to-many relationships cannot be represented directly by
relations in the same way that one-to-one and one-to-many relationships are. The reason being, we cannot put multiple values in
one cell, but it’s understandable that many students correspond too many classes.
5.8 Define the term “foreign key” and give an example.


Foreign key: An attribute that is a key of one or more relations
other than the one in which it appears.
Ex: When you place student ID into the enrollment table. The
“student ID” in the enrollment table is the foreign key.
5.9 Show a way to represent the 1:1 relationship in your answer to
question 5.7. Use data structure diagrams.
John Smith
IT Dept
Cubicle Number 10
John Smith
5.10 For your answer to question 5.9, describe a method for obtaining data
about one of the entities, given the key of the other.
06
Cubicle Number 10
John Smith
IT Dept
06
5.11Code the SQL statements to create a join having all data about both relations
from your work for question 5.9
SELECT
FROM
WHERE
*
STUDENT,LOCKER
SUDENT.StudentID = LOCKER.StudentID;
5.12. Why are some 1:1 relationships considered suspicious?
Under what conditions should relations in a 1:1 relationship be
combined into 1 relation?
Some 1:1 relationships are considered suspicious because they have hash marks that indicate the relationship is mandatory
in both directions. Conditions where they should be combined into one relation are when both entities have the same key.
1.13 Define the terms parent and child and give a definition of each. The
terms parent and child can be applied to the 1:N relationship where the
parent is on the “one” and the child is on the “many” side.
In the above diagram, professor would be the “parent”, and
student would be the “child”.
1.14 Show how to represent the 1:N relationship in your answer to question 5.7.
Use a data structure diagram.
Class
Student
Primary Key
ClassID
StudentID
ClassName
ClassID
Location
StudentName
Foreign Key
5.15 For your answer to question 5.14, describe a method for obtaining data for all of the children,
given the key of parent. Describe a method for obtaining data for the parent given a key of the
child.


Given a class ID, we can find all the students in that class.
Given the child key, we can find what class a student has.
5.16 . For your answer to question 5.14, code a SQL statement that creates a relation having all
data from both tables.
SELECT
FROM
WHERE
*
Class, Student
Class.ClassID = Student.ClassID;
5.17 For a 1:N relationship, explain why you must place the key of the parent in the child, rather
than placing the key of the child in the parent.
In 1:N relationships, the parent relation is the “one” and the child relation is the “many”. Because
attributes in a relation can have only a single value, each Child record has room for only one
Parent record. The key in placed in the Child because the Parent is not able to contain multiple
values for the many Child records.
5.18 Give examples of binary 1:N relationships, other than those in this text, for an optionalto-optional relationship, an optional-to-mandatory relationship, a mandatory-to- optional
relationship, and a mandatory-to-mandatory relationship Illustrate your answer using data
structure diagram.
Class
ClassID
ClassName
Location
Student
StudentID
Schedule
StudentName
Optional-to-optional because a class can be
created before students actually enroll and students
are not required to be enrolled in that class.
5.18 cont’d
Class
ClassID
ClassName
Location
Student
StudentID
Schedule
StudentName
Optional-to-mandatory relationship, ClassName is optional
and StudentID is mandatory to distinguish between other
students.
5.18 cont’d
Class
ClassID
ClassName
Location
Student
StudentID
Schedule
StudentName
Mandatory-to- optional relationship, a ClassID is mandatory
to distinguish between other classes and StudentName is
optional because that student is not required to be in that
class.
5.18 cont’d
Class
ClassID
ClassName
Location
Student
StudentID
Schedule
StudentName
Mandatory-to-mandatory relationship, ClassID and StudentID
are both mandatory to determine which student is in that class
and what class that student is enrolled.
5.19 Show how to represent the N:M relationship in your answer to question 5.7. Use a data structure
diagram.
Student
StudentID
Classes
StudentName
Schedule
ClassDesc
Student-Class
StudentID
Schedule
Student to Student-Class and Classes to
Student-Class are in essence a 1:N
relationship
5.20 For your answer to question 5.19, describe a method for obtaining the children for one entity, given the key
of the other. Also, describe a method for obtaining the children for the second entity, given the key of the first.


Given a StudentID, we can look up the appropriate row
in Student-Class and the Schedule of that Student.
Since we obtained the Schedule, we can look up the
ClassDesc (classes) that our Student has in his
Schedule.
2.21 For your answer to question 5.19, code a SQL statement that
creates a relation having all data from all tables.
SELECT
FROM
WHERE
AND
*
CLASSES, STUDENT_CLASS, STUDENT
CLASSES.Schedule = STUDENT_CLASS.Schedule
STUDENT_CLASS.StudentID = STUDENT.StudentID
5.22 Why is it not possible to represent a N:M relationship with the same
strategy used to represent 1:N relationship.
•Because in a 1:N relationship we are placing the foreign key
from one table into the other. (Note: in a 1:N the foreign key
always goes into the many-side of the relationship.
•In a N:M relationship we are creating a new relation called an
intersection relation, which has a composite key consisting of
the keys from each of the tables that formed it.
5.23 Explain the meaning if the term intersection relation.
This is when a new relation is made from the keys from each of the tables that
formed it.
5.24 Define 3 types of recursive binary relationships and give an example of each.


A recursive relationship is a relationship that a relation has with itself.
Recursive relationships adhere to the same rules as the binary
relationships.
 1:1 and 1:M relationships are saved using foreign keys
 M:N relationships are saved by creating an intersecting relation
Student
StudentID
ProfessorID
StudentName
ProfessorID is a
Foreign Key referencing
the Primary Key StudentID
5.26 Code a SQL statement that creates a table with all columns from the parent and child tables in your
answer to question 25.
SELECT
FROM
WHERE
*
Student
Student.ProfessorID = Student.StudentID;
5.27 Show how to represent the 1:N recursive relationship in your answer to question 24. How does this differ from the
representation of 1:N non-recursive relationships?
-A recursive relationship is a relationship that a relation has with itself.
5.28 Code a SQL statement that creates a table with all columns from the
parent and child tables in your answer to question 27.
SELECT
FROM
WHERE
*
Student
Student.ProfessorID = Student.StudentID;
5.29 Show how to represent the M:N recursive relationship in your answer to
question 24. How does this differ from the representation of M:N non-recursive
relationships?
-A recursive relationship is a relationship that a relation has with itself.
This Concludes Our Presentation on
Chapter 5 “Getting Started With
Database Technology”
Thank You for Choosing:
Team 1 Consultants:
University
of
Jonathan Jackson
Houston
Ronald Braun
co. 2005
Kirby Mack
Michael Runfola
Travis Riley
Ben Wessing