Microsoft Access 2003
Relationships
Email: training@health.ufl.edu
Web site: http://training.health.ufl.edu
Microsoft Access: Relationships
2.0 hours
This workshop requires completion of "Access: Basics" and "Access: Tables" highly
recommended. Topics include one-to-one and one-to-many relationships; junction tables;
primary and foreign keys; cascade update and cascade delete; and join properties.
Topics:
- Relationship Presentation
- Relational Databases
- Design Rules
- Primary Keys
- Enforcing referential integrity
- One to One
-
One to Many
Many to Many (indeterminate)
Cascade Update
Cascade Delete
Relationship Issues
Worksheets:
Relationships ............................................................................................................................. 1
One to Many ......................................................................................................................... 1
One to One ............................................................................................................................ 1
Many to Many ....................................................................................................................... 2
Edit Relationships ..................................................................................................................... 3
Enforce Referential Integrity ................................................................................................ 3
Cascade Update Related Records ......................................................................................... 4
About Relationships in Databases (Access Help) ..................................................................... 6
How do relationships work? ................................................................................................. 6
A One-to-Many Relationship ................................................................................................ 7
A Many-to-Many Relationship ............................................................................................. 7
A one-to-one relationship ..................................................................................................... 8
Defining relationships ........................................................................................................... 8
Pandora Rose Cowart
Training Specialist
Information Technology Center
Health Science Center
PO Box 100152
Gainesville, FL 32610-0152
352-273-5051
prcowart@ufl.edu
http://training.health.ufl.edu
Updated: 1/4/06
Page 1
Relationships
You can view the relationships of your database by clicking on the Relationships ( ) button while
looking at the database window, or by choosing Relationships... from the Tools menu.
One to Many
The most common relationship you will create will be a one-to-many relationship. This will reflect a
link between a Primary Key field and a non Primary Key field. Primary Keys are discussed in the
PowerPoint presentation.
Example: One person can visit many times. (Jack Johnson visited on 1/23/03, 1/31/03, and 3/10/03.)
Person ID
P1
P2
P3
First Name
Jack
Julie
Jeffery
Person ID
First Name
Last Name
Address
City
Zip
Home Phone
Work Phone
1
Last Name
Johnson
Jacobs
Jackson

Address
PO Box 123
PO Box 456
PO Box 789
Person ID
Visit ID
Visit Date
Comments
Person ID
P1
P2
P1
P1
P3
VisitID
V1
V2
V3
V4
V5
VisitDate
1/23/03
1/23/03
1/31/03
3/10/03
2/15/03
Comments
First Contact
First Contact
Gave Card
Likes hot tea
Spouse Joan
Each visit has its own unique comments, so one
unique person, has many unique visits.
One to One
The other relationship you can create in Access is a one-to-one relationship. This will reflect a link
between a Primary Key field and another Primary Key field.
Example: One person can have one spouse. (Jack Johnson is married to Mary Johnson.)
Person ID
P1
P2
P3
First Name
Jack
Julie
Jeffery
Person ID
First Name
Last Name
Address
City
Zip
Home Phone
Work Phone
1
Last Name
Johnson
Jacobs
Jackson
1
Address
PO Box 123
PO Box 456
PO Box 789
PersonID
Spouse First
Spouse Last
Person ID
P1
P3
Spouse First
Mary
Mark
Spouse Last
Johnson
Jacobs
One-to-one relationships can be merged into one
table. When the tables become too large, or the
data you want to track does not apply to a majority
of your records, you may wish to pull that data out
into its own table, as with this example.
Updated: 1/4/06
Page 2
Many to Many
Often you will find that you have a many-to-many relationship.
Example: One student can have many tutors
(Jack Johnson is tutored by Mary McGuire, Mark Madden, and Misty Martin)
Student ID
S1
S2
S3
S4
First Name
Jack
Julie
Jeffery
John
Last Name
Johnson
Jacobs
Jackson
Jenson
Address
PO Box 123
PO Box 456
PO Box 789
PO Box 021
Tutor ID
T1
T2
T3
T4
T5
First Name
Mary
Matt
Misty
Melanie
Mark
Last Name
McGuire
Madden
Martin
Masters
Madden
Office#
3-045
3-041
3-048
3-053
3-054
Last Name
McGuire
Madden
Martin
Masters
Madden
Office#
3-045
3-041
3-048
3-053
3-054
but each tutor can support many students
(Mary McGuire tutors Jack Johnson, Julie Jacobs, and John Jenson)
Student ID
S1
S2
S3
S4
First Name
Jack
Julie
Jeffery
John
Last Name
Johnson
Jacobs
Jackson
Jenson
Address
PO Box 123
PO Box 456
PO Box 789
PO Box 021
This kind of relationship is too complicated. It
requires the intervention of a junction table to
keep track of individual occurrences.
The session table pulls the students and the
tutors into unique records by keeping track of
the date and time of each session.
1 Student, Many Tutors
Jack Johnson
Mary, 2/14, 2pm
Mark, 2/14, 5pm
Misty, 2/15, 5pm
1 Tutor, Many Students
Mary McGuire
Jack, 2/14, 2pm
Julie, 2/14, 4pm
John, 2/14, 6pm
Tutor ID
T1
T2
T3
T4
T5

Student ID
First Name
Last Name
Address
City
Student ID
First Name
Last Name
Address
City
First Name
Jack
Julie
Jeffery
John
Updated: 01/04/06
Stud ID
S1
S2
S1
S4
S1
Sess ID
1
2
3
4
5
Date/Time
2/14, 2pm
2/14, 4pm
2/14, 5pm
2/14, 6pm
2/15, 5pm
 TutorID
First Name
Last Name
Office#
1
1

Student ID
S1
S2
S3
S4
First Name
Mary
Matt
Misty
Melanie
Mark
Session ID
Student ID
Tutor ID
Date/Time
Tutor ID
T1
T1
T5
T1
T3
TutorID
First Name
Last Name
Office#

Tutor ID
T1
T2
T3
T4
T5
First Name
Mary
Matt
Misty
Melanie
Mark
Page 3
Edit Relationships
You will see this Edit Relationships window when you create the relationship, by dragging a field from
one table to the field of another table or when you double-click on an existing relationship.
The relationship type will depend on the status of the original fields.
- If you drag a primary key field to a non-primary key field, you will see One-To-Many.
-
If you drag a primary key field to another primary key field, you will get a One-To-One
relationship.
-
If you drag a non-primary key field to another non-primary key field you will see Indeterminate.
You cannot Enforce Referential integrity with an indeterminate relationship type.
Enforce Referential Integrity
Access Help – "Referential integrity is a system of rules that Microsoft Access uses to ensure that
relationships between records in related tables are valid and that you don't accidentally delete or change
related data.
Select this check box to enforce referential integrity for this relationship only if all the following apply:
- The matching field from the primary table is a primary key or has a unique index
- The related fields have the same data type
- and Both tables are stored in the same Access database
Clear this check box to allow changes to related tables that would break referential integrity rules."
Enforcing referential integrity applies a set of rules to your tables that will help make your tables have
the correct information. This ensures you will not be able to sell anything to Mr. Smith, unless he exists
in your customer table.
Once you have enforced the referential integrity, you have two new choices, Cascade Update Related
Records and Cascade Delete Related Records.
Updated: 1/4/06
Page 4
Cascade Update Related Fields
Once you enforce referential integrity, Access won’t allow you to change the linked field in either table.
If you check the “Cascade Update Related Records”, you will be allowed to change the data in the main
table (the table where the field is a primary key). The only field that should be in both tables is the
Primary Key from the main table. So the Cascade Update we are talking about is effecting the Primary
Key fields only.
Example: We need to change Johnson to Johnston.
One-To-Many relationship, with enforced referential integrity but no cascade update.
VISIT TABLE
Last Name
Johnson
Jacobs
Johnson
Johnson
Jackson
VisitID
V1
V2
V3
V4
V5
Person Table
VisitDate
1/23/03
1/23/03
1/31/03
3/10/03
2/15/03
Comments
First Contact
First Contact
Gave Card
Likes hot tea
Spouse Joan
First Name
Jack
Julie
Jeffery
Last Name
Johnson
Jacobs
Jackson
Address
PO Box 123
PO Box 456
PO Box 789
If you try to change Johnson to Johnston in the Visit table access will say:
If you try to change Johnson to Johnston in the Person table Access will say:
Solution: Create a new record in the Person table for Mr. Johnston, then change all of the Johnsons to
Johnston in the Visit table, and then remove the Johnson record from the Person table.
Or turn on the Cascade Update Related Fields.
If you try to change Johnson to Johnston in the Visits table access will give you the same error as above,
however if you try to change Johnson to Johnston in the Person table Access will let you change the
data, no questions asked, and it will automatically change all the Johnsons to Johnstons in the Visits
table.
Updated: 01/04/06
Page 5
Cascade Delete Related Records: Once you enforce referential integrity, Access won’t allow you to
delete the primary key field in the main table. If you check the “Cascade Delete Related Records”, you
will be allowed to delete the data in the main table, and it will in turn, cascade delete the related records.
Example: We need to Delete Johnson
One-To-Many relationship, with enforced referential integrity but no cascade delete.
VISITS
PEOPLE
Last Name
Johnson
Jacobs
Johnson
Johnson
Jackson
VisitID
V1
V2
V3
V4
V5
VisitDate
1/23/03
1/23/03
1/31/03
3/10/03
2/15/03
Comments
First Contact
First Contact
Gave Card
Likes hot tea
Spouse Joan
First Name
Jack
Julie
Jeffery
Last Name
Johnson
Jacobs
Jackson
Address
PO Box 123
PO Box 456
PO Box 789
You can delete the records in the Visits table with no problem. There doesn’t have to be matching
records in the Visits table for each person in the People table. However, if you try to delete Johnson
from the People table, you will get this message:
If you turn on the Cascade Delete Related Records, you will be able to delete Johnson from the People
table, but it will delete all the related records in the Visits table.
VISITS
Last Name
Johnson
Jacobs
Johnson
Johnson
Jackson
PEOPLE
VisitID
V1
V2
V3
V4
V5
VisitDate
1/23/03
1/23/03
1/31/03
3/10/03
2/15/03
Comments
First Contact
First Contact
Gave Card
Likes hot tea
Spouse Joan
First Name
Jack
Julie
Jeffery
Last Name
Johnson
Jacobs
Jackson
Address
PO Box 123
PO Box 456
PO Box 789
Updated: 1/4/06
Page 6
About Relationships in Databases (Access Help)
After you've set up different tables for each subject in your database, you need a way of telling
Microsoft Access how to bring that information back together again. The first step in this process is to
define relationships between your tables. After you've done that, you can create queries, forms, and
reports to display information from several tables at once. For example, this form includes information
from five tables:
How do relationships work?
In the previous example, the fields in five tables must be coordinated so that they show information
about the same order. This coordination is accomplished with relationships between tables. A
relationship works by matching data in key fields, usually a field with the same name in both tables. In
most cases, these matching fields are the primary key from one table, which provides a unique identifier
for each record, and a foreign key in the other table. For example,
employees can be associated with orders they're responsible for
by creating a relationship between the Employees table and the
Orders table using the EmployeeID fields.
Updated: 01/04/06
Page 7
DEFINITIONS
Relationship: An association established between common fields (columns) in two tables. A
relationship can be one-to-one, one-to-many, or many-to many.
Primary Key: One or more fields whose value or values uniquely identify each record in a table. In a
relationship, a primary key is used to refer to specific records in one table from another table. A
primary key is called a foreign key when it is referred to from another table.
Foreign Key: One or more table fields to the primary key field or fields in another table. A foreign
key indicates how the tables are related – the data in the foreign key and primary key fields must match.
A One-to-Many Relationship
A one-to-many relationship is the most
common type of relationship. In a oneto-many relationship, a record in Table
A can have many matching records in
Table B, but a record in Table B has
only one matching record in Table A.
A Many-to-Many Relationship
In a many-to-many relationship, a record in Table A
can have many matching records in Table B, and a
record in Table B can have many matching records
in Table A. This type of relationship is only possible
by defining a third table (called a junction table)
whose primary key consists of two fields, the
foreign keys from both Tables A and B. A many-tomany relationship is really two one-to-many
relationships with a third table. For example, the
Orders table and the Products table have a many-tomany relationship that's defined by creating two
one-to-many relationships to the Order Details table.
Updated: 1/4/06
Page 8
A one-to-one relationship
In a one-to-one relationship, each record in Table A can
have only one matching record in Table B, and each
record in Table B can have only one matching record in
Table A. This type of relationship is not common,
because most information related in this way would be in
one table. You might use a one-to-one relationship to
divide a table with many fields, to isolate part of a table
for security reasons, or to store information that applies
only to a subset of the main table. For example, you
might want to create a table to track employees
participating in a fundraising soccer game.
Defining relationships
You define a relationship by adding the tables you want to relate to the Relationships window, and then
dragging the key field from one table and dropping it on the key field in the other table.
The kind of relationship that Microsoft Access creates
depends on how the related fields are defined:



A one-to-many relationship is created if only one of
the related fields is a primary key or has a unique
index.
A one-to-one relationship is created if both of the
related fields are primary keys or have unique
indexes.
A many-to-many relationship is really two one-tomany relationships with a third table whose primary
key consists of two fields – the foreign keys from the
two other tables.
Note If you drag a field that isn't a primary key and doesn't have a unique index to another field that
isn't a primary key and doesn't have a unique index, an indeterminate relationship is created. In queries
containing tables with an indeterminate relationship, Microsoft Access displays a default join line
between the tables, but referential integrity won't be enforced, and there's no guarantee that records are
unique in either table.
Updated: 01/04/06
Page 9
Updated: 1/4/06
Page 10
Relationship Error Messages
There are four reasons you will not be able to enforce referential integrity on your relationships.
(1) No primary key set.
(2) Conflicting Data Types
You are linking a text field to a number field. Or date to text, or possibly even a integer value to
a double value. You need to make sure the data in both fields is the same type. (If linking
AutoNumber to a Number, then the Number should be a LONG INTEGER).
(3) You have one of the tables open.
May possibly be because you also have a query, form or report, based on one of the tables, open.
Cancel the relationship window, close the object, and try again.
(4) Mismatched data
You have created data in a table (most likely the "MANY" table) that doesn't exist in the other.
You need to either place the data in both tables, or remove the offending data. If needed, you
can run a FIND MISMATCHED query to help find the data that isn't matching.
Updated: 01/04/06