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Web-Enabled Decision Support SystemsRelationships: Linking Access Tables

Prof. Name name@email.comPosition (123) 456-7890University Name

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Overview 7.1 Introduction 7.2 Defining Relationships in the Relationships Window 7.3 Types of Relationships 7.4 Hands-On Tutorial: Defining Relationships in Access 7.5 Working with Existing Relationships 7.6 In-Class Assignment 7.7 Summary

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Introduction A relationship connects or relates two tables Categorize relationships by:

– Cardinality One-to-one, many-to-one, and many-to-many

– Degree Unary, binary, and ternary

The Relationships Window is used to define relationships– Add tables– Relate tables– Specify relationship types– Enforce referential integrity

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Introduction (cont.)

Conversion from ER Diagram to Access Relationships Window:

Register

Grade

COURSESTUDENTEntity-Relationship Diagram

Relational Schema

Relationships Windows

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Overview 7.1 Introduction 7.2 Defining Relationships in the Relationships Window 7.3 Types of Relationships 7.4 Hands-On Tutorial: Defining Relationships in Access 7.5 Working with Existing Relationships 7.6 In-Class Assignment 7.7 Summary

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The Relationships Window The Relationships Window serves as a powerful graphical tool for

defining relationships

Database Tables

Relationships

The Access Relationships Window

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Opening the Relationships Window How-to: Open the Relationships Window

1. Open the university database file for this chapter.

2. From the active Database Window, click the Relationships button on the toolbar.

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Adding Tables to Relationships Window How-to: Add a Table

1. Click the Show Table button on the toolbar to open the Show Table dialog box.

2. Select the desired tables and click the Add button. 3. To return to the Relationships Window, click the Close button.

Show Table Dialog Box

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Exploring the Relationships Toolbar and Menu The Relationships Window has a number of frequently used features

accessible through toolbar buttons and the Relationships menu

Buttons Name Main Menu Function

Show Table Relationships | Show Table Opens the Show Table dialog box.

Show Direct Relationships

Relationships | Show Direct Displays the relationships for the selected table.

Show All Relationships

Relationships | Show All Displays all the relationships.

Clear Layout Edit | Clear Layout Clears the layout by removing tables and relationships from the display. This does not alter relationship definitions.

- - Relationships | Hide Table Temporarily hides the selected table.

- - Relationships | Edit Relationships

Opens the Edit Relationships dialog box for selected relationships.

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Drawing a Relationship Line Drawing a line between two tables defines a relationship

The process of drawing a line can be divided into two sub-tasks:1. Deciding the common field from two tables to draw the line between the

two fields. 2. Drawing the line and enforcing referential integrity constraints

Drawing a Relationship Line

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Referential Integrity A referential integrity constraint ensures that foreign key values of a

relation must come from the primary key values of the related relation– Otherwise, the value of a foreign key must be NULL

Table types:– Parent tables are tables with primary keys– Child tables are tables with foreign keys

An orphan record is a record for which there is no related parent record– Referential integrity constraints prevent the creation of orphan records

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Enforcing Referential Integrity How-to: Enforce Referential Integrity

1. Open the Show Table dialog box and add the department table to the Relationships Window.

2. Drag the DeptID field from one table to the DeptID field of the other table. 3. Check the Enforce Referential Integrity check box.

Enforcing Referential Integrity Using Edit Relationships Dialog Box

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Enforcing Referential Integrity (cont.)

How-to: Enforce Referential Integrity on an Existing Relationship1. Right-click on a relationship and choose the Edit Relationships option to

open the Edit Relationships dialog box.2. Check the Enforce Referential Integrity check box.

Observe:– The type of relationship is automatically detected and displayed at the

bottom of the Edit Relationships dialog box.– The “1” and “∞” symbols indicating a relationship type are displayed in the

Relationship Window. – Two other options related to cascade updates and cascade deletes

become available.

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Conditions Before Enforcement of Referential Integrity

Following are the few conditions that must hold before the enforcement of referential integrity:

– Rule 1: The common field on one side of the relationship must be a primary key field or must have a unique index.

– Rule 2: Linking fields must be of the same data type. The AutoNumber and Number data types constitute the only exceptions,

provided that their field size property is set to ReplicationID.

– Rule 3: The existing data in the tables cannot violate the referential integrity constraint.

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Conditions After Enforcement of Referential Integrity

Following are the few conditions that must hold after the enforcement of referential integrity:

– Rule 4: We cannot delete a record from the parent table if a record exists in the child table with the same key value.

– Rule 5: Similarly, we cannot update the common field from the parent table if corresponding records exist in the child table.

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Cascade Updates and Cascade Deletes Maintaining the last two conditions related to the referential integrity

can be tedious – Upon enforcement of the referential integrity constraints in the Edit

Relationships dialog box, the Cascade Update/Delete Related Fields options become available

Enforcing Cascade Updates and Cascade Deletes

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Cascade Updates and Cascade Deletes (cont.)

When the Cascade Delete Related Fields option is checked: – Cascade deletes are allowed

If we delete a record with a unique field in the parent table, Access automatically deletes the related records in the child table

– Must be very cautious whenever we delete records If we have not properly examined the relationship, we could mistakenly delete a

large amount of useful data

When the Cascade Update Related Fields option is checked: – Cascade updates are allowed

If we update a unique field in the parent table, Access automatically updates the related records in the child table

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Saving a Layout of Relationships Window Access automatically saves relationship lines that have been added to

the Relationships Window– Access does not automatically preserve its layout– We must manually save it after it has been created or altered– Saving the layout preserves the orientation of the field lists and so

preserves the readable view of the Relationships Window

How-to: Save the Layout of Relationships Window– Choose the File | Save option from the Main menu.

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Overview 7.1 Introduction 7.2 Defining Relationships in the Relationships Window 7.3 Types of Relationships 7.4 Hands-On Tutorial: Defining Relationships in Access 7.5 Working with Existing Relationships 7.6 In-Class Assignment 7.7 Summary

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Types of Relationships Three types of relationships can exist among database tables:

– One-to-Many or Many-to-One: A record in Table A can have many matching records in Table B, but a record

in Table B has exactly one matching record in Table A Most common relationship type

– Many-to-Many: Consists of two one-to-many relationships, one from Table A to Table B and

one from Table B to Table A

– One-to-One: A record in Table A has exactly one matching record in Table B, and a record

in Table B has exactly one matching record in Table A

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Access Relationship Assignment When Access automatically detects the relationship types, it refers to

the following predefined rules for an assignment:

– If the common field in the parent table is the primary key or has a unique index and the corresponding field in the child table is a foreign key:

Access establishes a One-to-Many relationship between these two tables

– If a common field from both the tables is the primary key or has a unique index:

Access establishes a One-to-One relationship between these two tables

– If none of the common fields is a primary key or has a unique index: Access establishes an indeterminate relationship between these two tables We cannot enforce referential integrity constraints for such relationships

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Overview 7.1 Introduction 7.2 Defining Relationships in the Relationships Window 7.3 Types of Relationships 7.4 Hands-On Tutorial: Defining Relationships in Access 7.5 Working with Existing Relationships 7.6 In-Class Assignment 7.7 Summary

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Hands-On Tutorial: Defining Relationships in Access

How-to: Define Relationships in Access

1. Open the university database and open the Relationships Window by choosing the Tools | Relationships option from the Main menu.

2. Right-click anywhere in the window and choose the Show Table option to display the Show Table dialog box.

3. Make sure that all of the tables from the university database are visible in the Relationships Window. Click the Close button to return to the Relationships Window.

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Hands-On Tutorial: Defining Relationships in Access (cont.)

4. Relate the tblCollege and tblDepartment tables: Select the CollegeID field from tblCollege; drag-and-drop it on the CollegeID

field in tblDepartment to open the Edit Relationships dialog box.

5. Enforce the referential integrity, cascade updates related fields, and cascade deletes related records by checking appropriate check boxes.

6. Complete relationships for the university database by repeating steps 4 and 5 for the following pairs of tables: tblCourse and tblSection tblSection and tblFaculty tblFaculty and tblDepartment tblTranscript and tblSection.

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Overview 7.1 Introduction 7.2 Defining Relationships in the Relationships Window 7.3 Types of Relationships 7.4 Hands-On Tutorial: Defining Relationships in Access 7.5 Working with Existing Relationships 7.6 In-Class Assignment 7.7 Summary

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Working with Existing Relationships Because data and data constraints can change, we may also want to

modify the existing relationships in our databases– Adding/Removing tables– Modifying/Removing relationships

We use the Relationships Window to make these modifications – Before opening the Relationships Window to institute changes, we must be

sure to close any open tables in the Database Window – Prevents any discrepancy between what we see in the table Design View

and what exists in the Relationships Window

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Adding, Removing, and Hiding Tables in Relationships Window

We add database tables to the Relationships Window using the Show Table dialog box

If we would like to remove a table from the Relationships Window, we select the table and press Delete on the keyboard – Note that removing a table from the Relationships Window does not

remove it from the database– It only affects the layout of the Relationships Window

We can also hide tables in the Relationships Window – To hide a table, select the table and choose the Relationships | Hide Table

option from the Main menu– To unhide the hidden tables, choose the Relationships | Show Table option

from the Main menu

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Editing Relationships Relationship lines are not permanent

– We can edit or remove these lines as we build our database

How-to: Edit a Relationship1. Open the Relationships Window.

If necessary, unhide/add the missing tables or rearrange the existing tables for a clearer view.

2. Locate and double-click on the relationship line that we would like to edit to bring up the Edit Relationships dialog box.

3. Make the desired changes in this dialog box to edit the relationship.

4. Save the relationships layout.

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Deleting Relationships How-to: Delete a Relationship

1. Open the Relationships Window; locate and select the relationship we would like to delete.

2. Press Delete on the keyboard.

3. Click Yes when Access seeks confirmation before permanently deleting the relationship.

4. Save the relationships layout.

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Overview 7.1 Introduction 7.2 Defining Relationships in the Relationships Window 7.3 Types of Relationships 7.4 Hands-On Tutorial: Defining Relationships in Access 7.5 Working with Existing Relationships 7.6 In-Class Assignment 7.7 Summary

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In-Class Assignment Open the University.mdb database from the “Chapter7” folder on the

book website Examine tblStudent, tblCourse, tblStudentCourse, tblInstructor,

tblInstructorCourse, and tblRoom tables

Answer the following questions:1. What is the relationship between the table tblStudent and tblCourse?

Why?2. Open the Relationships Window and show all tables. Create relationships

between the tblStudent and tblCourses tables. Enforce the referential integrity.

3. What types of relationships exist between tblStudent, tblCourse, and tblStudentCourse? Add these relationships in the Relationships Window.

4. Edit relationships so that when student information is changed, the changes are automatically reflected in all related tables.

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Overview 7.1 Introduction 7.2 Defining Relationships in the Relationships Window 7.3 Types of Relationships 7.4 Hands-On Tutorial: Defining Relationships in Access 7.5 Working with Existing Relationships 7.6 In-Class Assignment 7.7 Summary

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Summary A relationship is an association between tables.

The graphical interface known as the Relationships Window allows us to create, view, and edit relationships.

A Referential integrity constraint is a constraint on data that prevents the creation of orphan records in tables.

When enabled, the Cascade Update and Delete options can update or delete records from the child table that are related to records in the parent table that have been updated or deleted, respectively.

Relationship types are denoted by the symbols 1 and ∞.

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Additional Links Review more sample Relationships Window from database files on the

book website: http://www.dssbooks.com.

Refer MS Access 2003: The Complete Reference by Virginia Andersen for more details on Relationships.

Also refer Access 2003 Bible by Cary N. Prague, Michael R. Irwin, and Jennifer Reardon.

Also see: http://msdn.microsoft.com/ for useful tips on Relationships