Table Relationships
• Establishing table relationships is the way we
draw together data from separate tables. This
minimizes redundant data and maintains higher
levels of database reliability.
• Types of relationships
– One-to-One
– One-to-Many
– Many-to-Many
Homework Reminder
• First step was to develop list of tables and fields
and have a partner look it over
– As part of your database project, I will be looking for a
minimum of three tables
– As you develop and refine the list of tables and fields,
it is useful to have copies of the intermediate steps
with some annotation describing why you did
particular refinements. This will help you in the
project to describe why you made particular choices
• Second step is to create your tables in access. I
won’t be collecting this as homework, but this is
part of the process of developing your database
project
Table Relationships
One-to-One
Student
ID
First
Name
Last
Name
Student
ID
A111
A111
A112
A112
A113
A113
A114
A114
A115
A115
A116
A116
Social Security
Number
Phone
number
Table Relationships
• One-to-one relationships are relatively rare in most database
structures. Further, one-to-one relationships can be thought of as a
specialized case of one-to-many.
• The most common type of “true” one-to-one relationship is when one
table is a SUBSET TABLE of another. Example- faculty are a subset
of all university staff members. Graduate students are also a subset
of all university staff. Thus, one way of implementing a personnel
database would be to have a staff table (with information common to
all staff members such as social security number), and separate
faculty and graduate student tables to hold data that applies only to
each of these specialized type of staff members.
• When you have a one-to-one relationship, the linkage is achieved by
having the same primary key in both tables (e.g., social security
number in the staff table, and in the faculty or graduate student
table).
Table Relationships
One-to-Many
Student
ID
First
Name
Last
Name
Student ID
Class
A111
A111
FW893
A112
A111
FW100
A113
A111
FW205
A114
A113
FW893
A115
A113
FW205
A116
A114
FW999
…
Table Relationships
One-to-Many
Example-Many fish are caught at each site
Site Table
Stream Name
Position
Site
Year
Month
Day
Width
Depth
Temperature
Conductivity
Individual Fish Table
Stream Name
(CPK/FK)
Position
(CPK/FK)
Site
(CPK/FK)
Year
(CPK/FK)
Month (CPK/FK)
Day
(CPK/FK)
Fish_ID (CPK)
Species Code
Length
Age
(CPK)
(CPK)
(CPK)
(CPK)
(CPK)
(CPK)
Linkage is established by taking primary key (PK, or composite primary key CPK)
from “One” side of relationship, and inserting that into the “Many” side of the
relationship. This now becomes what is known as a “FOREIGN KEY”.
Many-to-Many Relationships
Student Table
Student_ID (PK)
First Name
Last Name
Street
City
State
Zipcode
Phone
Class Table
Class_ID (PK)
Class Name
Class Description
Instructor_ID
Max Enrollment
Problem here is how to link Students with classes. Each student
can take multiple classes, and each class has multiple students.
Rectifying Many-to-Many Relationships
Student Table
Student_ID (PK)
First Name
Last Name
Street
City
State
Zipcode
Phone
Enrollment
Table
Student_ID (CPK/FK)
Class_ID (CPK/FK)
Class Table
Class_ID
(PK)
Class Name
Class Description
Instructor_ID
Max Enrollment
Enrollment (???)
Notice that the Enrollment Table serves to link the Student Table and the Class Table.
As such, it has both of the primary keys from each of these tables as foreign keys.
The relationship between Student Table and Enrollment Table is One-to-Many
(because each student can take many classes, but there is only one Student_ID,
Class_ID combination in the Student-Class Table for each student
Class Table and Enrollment Table is One to Many because each student can be
enrolled in a class only once
Table Relationships
Rectifying Many-to-Many
Student
ID
Class
Class
Course
Title
Instructor
A111
A111
FW893
FW100
Intro…
Habron
A112
A111
FW100
FW205
Principals
…
Riley
A113
A111
FW205
FW325
Ecosys…
Sarnelle
A114
A113
FW893
FW893
Seminar..
Hayes
A115
A113
FW205
FW899
MS res…
????
A116
A114
FW999
FW999
PhD res
????
Stude
nt ID
First
Name
Last
Name
Creating Relationships in Access
• Open Relationship Window (has three little
boxes connected with lines)
• Go to menu and select Relationships, then
Show Table
• Select Tables to add (usually all tables in
database), then close Show Table
• It’s handy to stretch each table display so
you can see all the fields
Creating Relationships in Access
• Usually I start with “biggest” tables
meaning the table with the largest unit of
observation. In this example, it is the
stream list and the sample list tables
• To create relationship between two tables
using a single key, simply click on field and
drag to same field in connected table
Creating Relationships in Access
• Note that in this example, Access “knows”
that the relationship is one to many
• Can click check box to Enforce Referential
Integrity
– This insures that a record must exist in the
“One” table before a corresponding record in
the “Many” table can be entered
– “Cascade” options help enforce referential
integrity
Creating Relationships in Access
• Cascade options
– “Cascade Update Related Fields”
automatically changes the primary key field
values in the “Many” table if you change the
primary key field values in the “One” table
(show example)
– “Cascade Delete Related Records”
automatically deletes related records in the
“Many” table if you delete a record in the
“One” table
Creating Relationships in Access
• Select “Join Type” – Note that this is a
critical decision and depends on your
“business rules”!!!
– Option 1 includes only records where both are
equal EQUIJOIN
– Option 2 includes only records where “One”
side exists
– Option 3 includes only records where “Many”
side exists
Join Types Illustration
Stream
ID
Stream
Name
1
…
Stream ID
Sample ID
Date
Au Gres
1
1
10-102004
2
Rifle
River
1
2
10-112004
3
Miners
Creek
1
3
10-122004
4
Albany
Creek
3
4
10-152004
3
5
10-162004
3
6
10-172004
Join Types Illustration
Option 1- both equal
Stream ID
Stream Name Sample ID
Date
1
Au Gres
1
10-102004
1
Au Gres
2
10-112004
1
Au Gres
3
10-122004
3
Miners Creek
4
10-152004
3
Miners Creek
5
10-162004
3
Miners Creek
6
10-172004
Join Types Illustration
Option 2- All Records from Primary Table
Stream ID
Stream Name
Sample ID Date
1
Au Gres
1
10-10-2004
1
Au Gres
2
10-11-2004
1
Au Gres
3
10-12-2004
2
Rifle River
3
Miners Creek
4
10-15-2004
3
Miners Creek
5
10-16-2004
3
Miners Creek
6
10-17-2004
4
Albany Creek
Join Types Illustration
Option 3- Doesn’t make sense when enforce
referential integrity because can’t have a
record in the “Many” table without a
corresponding record in “One” table