Home > Articles > Data > SQL

This chapter is from the book

Union

So far we’ve discussed finding the items that are common in two sets (intersection) and the items that are different (difference). The third type of set operation involves adding two sets (union).

Union in Set Theory

Union lets you combine two sets of similar information into one set. As a scientist, you might be interested in combining two sets of chemical or physical sample data. For example, a pharmaceutical research chemist might have two different sets of compounds that seem to provide a certain beneficial effect. The chemist can union the two sets to obtain a single list of all effective compounds.

Let’s take a look at union in action by examining two sets of numbers. The first set of numbers is as follows:

  • 1, 5, 8, 9, 32, 55, 78

The second set of numbers is as follows:

  • 3, 7, 8, 22, 55, 71, 99

The union of these two sets of numbers is the numbers in both sets combined into one new set:

  • 1, 5, 8, 9, 32, 55, 78, 3, 7, 22, 71, 99

Note that the values common to both sets, 8 and 55, appear only once in the answer. Also, the sequence of the numbers in the result set is not necessarily in any specific order. When you ask a database system to perform a UNION, the values returned won’t necessarily be in sequence unless you explicitly include an ORDER BY clause. In SQL, you can also ask for a UNION ALL if you want to see the duplicate members.

The members of each set don’t have to be just single values. In fact, you’ll probably deal with sets of rows when working with SQL.

To find the union of two or more sets of complex members, all the members in each set you’re trying to union must have the same number and type of attributes. For example, suppose you have a complex set like the one below. Each row represents a member of the set (a stew recipe), and each column denotes a particular attribute (an ingredient).

Potatoes

Water

Lamb

Peas

Rice

Chicken Stock

Chicken

Carrots

Pasta

Water

Tofu

Snap Peas

Potatoes

Beef Stock

Beef

Cabbage

Pasta

Water

Pork

Onions

A second set might look like the following:

Potatoes

Water

Lamb

Onions

Rice

Chicken Stock

Turkey

Carrots

Pasta

Vegetable Stock

Tofu

Snap Peas

Potatoes

Beef Stock

Beef

Cabbage

Beans

Water

Pork

Onions

The union of these two sets is the set of objects from both sets. Duplicates are eliminated.

Potatoes

Water

Lamb

Peas

Rice

Chicken Stock

Chicken

Carrots

Pasta

Water

Tofu

Snap Peas

Potatoes

Beef Stock

Beef

Cabbage

Pasta

Water

Pork

Onions

Potatoes

Water

Lamb

Onions

Rice

Chicken Stock

Turkey

Carrots

Pasta

Vegetable Stock

Tofu

Snap Peas

Beans

Water

Pork

Onions

Combining Result Sets Using a Union

It’s a small leap from sets of complex objects to rows in SQL result sets. When you’re dealing with rows in a set of data that you fetch with SQL, the attributes are the individual columns. For example, suppose you have a set of rows returned by a query like the following one. (These are recipes from John’s cookbook.)

Recipe

Starch

Stock

Meat

Vegetable

Lamb Stew

Potatoes

Water

Lamb

Peas

Chicken Stew

Rice

Chicken Stock

Chicken

Carrots

Veggie Stew

Pasta

Water

Tofu

Snap Peas

Irish Stew

Potatoes

Beef Stock

Beef

Cabbage

Pork Stew

Pasta

Water

Pork

Onions

A second query result set might look like this one. (These are recipes from Mike’s cookbook).

Recipe

Starch

Stock

Meat

Vegetable

Lamb Stew

Potatoes

Water

Lamb

Peas

Turkey Stew

Rice

Chicken Stock

Turkey

Carrots

Veggie Stew

Pasta

Vegetable Stock

Tofu

Snap Peas

Irish Stew

Potatoes

Beef Stock

Beef

Cabbage

Pork Stew

Beans

Water

Pork

Onions

The union of these two sets is all the rows in both sets. Maybe John and Mike decided to write a cookbook together, too!

Recipe

Starch

Stock

Meat

Vegetable

Lamb Stew

Potatoes

Water

Lamb

Peas

Chicken Stew

Rice

Chicken Stock

Chicken

Carrots

Veggie Stew

Pasta

Water

Tofu

Snap Peas

Irish Stew

Potatoes

Beef Stock

Beef

Cabbage

Pork Stew

Pasta

Water

Pork

Onions

Turkey Stew

Rice

Chicken Stock

Turkey

Carrots

Veggie Stew

Pasta

Vegetable Stock

Tofu

Snap Peas

Pork Stew

Beans

Water

Pork

Onions

Let’s assume you have a nice database containing all your favorite recipes. You really like recipes with either beef or onions, so you want a list of recipes that contain either ingredient. Figure 7-5 (on page 238) shows you the set diagram that helps you visualize how to solve this problem.

Figure 7-5

Figure 7-5 Finding out which recipes have either beef or onions

The upper circle represents the set of recipes that contain beef. The lower circle represents the set of recipes that contain onions. The union of the two circles gives you all the recipes that contain either ingredient, with duplicates eliminated where the two sets overlap. As you probably know, you first ask SQL to fetch all the recipes that have beef. In the second query, you ask SQL to fetch all the recipes that have onions. As you’ll see later, the SQL keyword UNION links the two queries to get the final answer.

By now you know that it’s not a good idea to design a recipes database with a single table. Instead, a correctly designed recipes database will have a separate Recipe_Ingredients table with one row per recipe per ingredient. Each ingredient row will have only one ingredient, so no one row can be both beef or onions at the same time. You’ll need to first find all the recipes that have a beef row, then find all the recipes that have an onions row, and then union them.

Problems You Can Solve with Union

A union lets you “mush together” rows from two similar sets—with the added advantage of no duplicate rows. Here’s a sample of the problems you can solve using a union technique with data from the sample databases:

  • “Show me all the customer and employee names and addresses.”
  • “List all the customers who ordered a bicycle combined with all the customers who ordered a helmet.”
  • “List the entertainers who played engagements for customer Bonnicksen combined with all the entertainers who played engagements for customer Rosales.”
  • “Show me the students who have an average score of 85 or better in Art together with the students who have an average score of 85 or better in Computer Science.”
  • “Find the bowlers who had a raw score of 155 or better at Thunderbird Lanes combined with bowlers who had a raw score of 140 or better at Bolero Lanes.”
  • “Show me the recipes that have beef together with the recipes that have garlic.”

As with other “pure” set operations, one of the limitations is that the values must match in all the columns in each result set. This works well if you’re unioning two or more sets from the same table—for example, customers who ordered bicycles and customers who ordered helmets. It also works well when you’re performing a union on sets from tables that have like columns—for example, customer names and addresses and employee names and addresses. We’ll explore the uses of the SQL UNION operator in detail in Chapter 10, “UNIONs.”

In many cases where you would otherwise union rows from the same table, you’ll find that using DISTINCT (to eliminate the duplicate rows) with complex criteria on joined tables will serve as well. We’ll show you all about solving problems this way using JOINs in Chapter 8, “INNER JOINs.”

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