Home > Articles > Data > Access

Working with Multiple-Table Queries in Access

Most database applications store their information in multiple tables. Being able to query and work with data from multiple tables requires some front-end planning to set up the tables correctly, and this chapter from Paul McFedries helps you to design databases to make such queries possible.
This chapter is from the book

In this chapter

  • Relational Database Fundamentals

  • Types of Relational Models

  • Enforcing Referential Integrity

  • Establishing Table Relationships

  • Working with Multiple Tables in a Query

  • Creating Other Types of Joins

  • Creating a Unique Values Query

  • Case Study

Most database applications (and all well-designed database applications) store their information in multiple tables. Although most of these tables have nothing to do with each other (for example, tables of customer information and employee payroll data), it's likely that at least some of the tables do contain related information (such as tables of customer information and customer orders).

Working with multiple, related tables in a query presents you with two challenges: You need to design your database so that the related data is accessible, and you need to set up links between the tables so that the related information can be retrieved and worked with quickly and easily in the query design window. This chapter tackles both challenges and shows you how to exploit the full multiple-table powers of Access.

Relational Database Fundamentals

Why do you need to worry about multiple tables, anyway? Isn't it easier to work with one large table instead of two or three medium-sized ones? To answer these questions and demonstrate the problems that arise when you ignore relational database models, take a look at a simple example: a table of sales leads.

The Pitfalls of a Nonrelational Design

Table 3.1 outlines a structure of a simple table (named Leads) that stores data on sales leads.

Table 3.1 A Structure of a Simple Sales Leads Table (Leads)

Field

Description

LeadID

The primary key.

FirstName

The contact's first name.

LastName

The contact's last name.

Company

The company that the contact works for.

Address

The company's address.

City

The company's city.

State

The company's state.

Zip

The company's ZIP code.

Phone

The contact's phone number.

Fax

The contact's fax number.

Source

Where the lead came from.

Notes

Notes or comments related to the sales lead.


This structure works fine until you need to add two or more leads from the same company (a not-uncommon occurrence). In this case, you end up with repeating information in the Company, Address, City, and State fields. (The Zip field also repeats, as do, in some cases, the Phone, Fax, and Source fields.)

All this repetition makes the table unnecessarily large, which is bad enough, but it also creates two major problems:

  • During data entry, the repeated information must be entered for each lead from the same company.

  • If any of the repeated information changes (such as the company's name or address), each corresponding record must be changed.

One way to eliminate the repetition and solve the data entry and maintenance inefficiencies is to change the table's focus. As it stands, each record in the table identifies a specific contact in a company. But it's the company information that repeats, so it makes some sense to allow only one record per company. You can then include separate fields for each sales lead within the company. The new structure might look something like the one shown in Table 3.2.

Table 3.2 A Revised, Company-Centered Structure of the Sales Leads Table

Field

Description

LeadID

The primary key.

Company

The company's name.

Address

The company's address.

City

The company's city.

State

The company's state.

Zip

The company's ZIP code.

Phone

The company's phone number.

Fax

The company's fax number.

First_1

The first name of contact #1.

Last_1

The last name of contact #1.

Source_1

Where the lead for contact #1 came from.

Notes_1

Notes or comments related to contact #1.

First_2

The first name of contact #2.

Last_2

The last name of contact #2.

Source_2

Where the lead for contact #2 came from.

Notes_2

Notes or comments related to contact #2.

First_3

The first name of contact #3.

Last_3

The last name of contact #3.

Source_3

Where the lead for contact #3 came from.

Notes_3

Notes or comments related to contact #3.


In this setup, the company information appears only once, and the contact-specific data (I'm assuming this involves only the first name, last name, source, and notes) appears in separate field groups (for example, First_1, Last_1, Source_1, and Notes_1). This solves the earlier problems, but at the cost of a new dilemma: The structure as it stands will hold only three sales leads per company. Of course, it's entirely conceivable that a large firm might have more than three contacts—perhaps even dozens. This raises two unpleasant difficulties:

  • If you run out of repeating groups of contact fields, new ones must be added. Although this might not be a problem for the database designer, most data-entry clerks generally don't have access to the table design (nor should they).

  • Empty fields take up as much disk real estate as full ones, so making room for, say, a dozen contacts from one company means that all the records that have only one or two contacts have huge amounts of wasted space.

How a Relational Design Can Help

To solve the twin problems of repetition between records and repeated field groups within records, you need to turn to the relational database model. This model was developed by Dr. Edgar Codd of IBM in the early 1970s. It was based on a complex relational algebra theory, so the pure form of the rules and requirements for a true relational database setup is quite complicated and decidedly impractical for business applications. The next few sections look at a simplified version of the model.

Step 1: Separate the Data

After you know which fields you need to include in your database application, the first step in setting up a relational database is to divide these fields into separate tables where the "theme" of each table is unique. In technical terms, each table must be composed of only entities (that is, records) from a single entity class.

For example, the table of sales leads you saw earlier dealt with data that had two entity classes: the contacts and the companies they worked for. Every one of the problems encountered with that table can be traced to the fact that we were trying to combine two entity classes into a single table. So the first step toward a relational solution is to create separate tables for each class of data. Table 3.3 shows the table structure of the contact data (the Contacts table) and Table 3.4 shows the structure of the company information (the Companies table). Note, in particular, that both tables include a primary key field.

Table 3.3 The Structure of the Contacts Table

Field

Description

ContactID

The primary key.

FirstName

The contact's first name.

LastName

The contact's last name.

Phone

The contact's phone number.

Fax

The contact's fax number.

Source

Where the lead came from.

Notes

Notes or comments related to the sales lead.


Table 3.4 The Structure of the Companies Table

Field

Description

CompanyID

The primary key.

CompanyName

The company's name.

Address

The company's address.

City

The company's city.

State

The company's state.

Zip

The company's ZIP code.

Phone

The company's phone number (main switchboard).


Step 2: Add Foreign Keys to the Tables

At first glance, separating the tables seems self-defeating because, if you've done the job properly, the two tables will have nothing in common. So the second step in this relational design is to define the commonality between the tables.

In the sales leads example, what is the common ground between the Contacts and Companies tables? It's that every one of the leads in the Contacts table works for a specific firm in the Companies table. So what's needed is some way of relating the appropriate information in Companies to each record in Contacts (without, of course, the inefficiency of simply cramming all the data into a single table, as we tried earlier).

The way you do this in relational database design is to establish a field that is common to both tables. You can then use this common field to set up a link between the two tables. The field you use must satisfy three conditions:

  • It must not have the same name as an existing field in the other table.

  • It must uniquely identify each record in the other table.

  • To save space and reduce data entry errors, it must be the smallest field that satisfies the two preceding conditions.

In the sales leads example, a field needs to be added to the Contacts table that establishes a link to the appropriate record in the Companies table. The CompanyName field uniquely identifies each firm, but it's too large to be of use. The Phone field is also a unique identifier and is smaller, but the Contacts table already has a Phone field. The best solution is to use CompanyID, the Companies table's primary key field. Table 3.5 shows the revised structure of the Contacts table that includes the CompanyID field.

Table 3.5 The Final Structure of the Contacts Table

Field

Description

ContactID

The primary key.

CompanyID

The Companies table foreign key.

FirstName

The contact's first name.

LastName

The contact's last name.

Phone

The contact's phone number.

Fax

The contact's fax number.

Source

Where the lead came from.

Notes

Notes or comments related to the sales lead.


When a table includes a primary key field from a related database, the field is called a foreign key. Foreign keys are the secret to successful relational database design.

Step 3: Establish a Link Between the Related Tables

After you have your foreign keys inserted into your tables, the final step in designing your relational model is to establish a link between the two tables. This step is covered in detail later in this chapter (see "Establishing Table Relationships").

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.

Overview


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information


To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.

Surveys

Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.

Newsletters

If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information


Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.

Security


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.

Children


This site is not directed to children under the age of 13.

Marketing


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information


If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.

Choice/Opt-out


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information


Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents


California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure


Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.

Links


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact


Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice


We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020