Home > Articles > Data > SQL Server

  • Print
  • + Share This
This chapter is from the book

SQL Set Operations

Now that you have a basic understanding of set operations, let's look briefly at how they're implemented in SQL.

Classic Set Operations versus SQL

As noted earlier, not many commercial database systems yet support set intersection (INTERSECT) or set difference (EXCEPT) directly. The current SQL Standard, however, clearly defines how these operations should be implemented. We think that these set operations are important enough to at least warrant an overview of the syntax.

As promised, we'll show you alternative ways to solve an intersection or difference problem in later chapters using JOINs. Because most database systems do support UNION, Chapter 10 is devoted to its use. The remainder of this chapter gives you an overview of all three operations.

Finding Common Values: INTERSECT

Let's say you're trying to solve the following seemingly simple problem.

  • "Show me the orders that contain both a bike and a helmet."

Translation

Select the distinct order numbers from the order details table where the product number is in the list of bike and helmet product numbers

Clean Up

Select the distinct order numbers from the order details table where the product number is in the list of bike and helmet product numbers

SQL

SELECT DISTINCT OrderNumber
FROM Order_Details
WHERE ProductNumber IN (1, 2, 6, 10, 11, 25, 26)

That seems to do the trick at first, but the answer includes orders that contain either a bike or a helmet, and you really want to find ones that contain both a bike and a helmet! If you visualize orders with bicycles and orders with helmets as two distinct sets, it's easier to understand the problem. Figure 7-6 shows one possible relationship between the two sets of orders using a set diagram.

Figure 7-6

Figure 7-6 One possible relationship between two sets of orders

Actually, there's no way to predict in advance what the relationship between two sets of data might be. In Figure 7-6, some orders have a bicycle in the list of products ordered, but no helmet. Some have a helmet, but no bicycle. The overlapping area, or intersection, of the two sets is where you'll find orders that have both a bicycle and a helmet. Figure 7-7 shows another case where all orders that contain a helmet also contain a bicycle, but some orders that contain a bicycle do not contain a helmet.

Figure 7-7

Figure 7-7 All orders for a helmet also contain an order for a bicycle.

Seeing "both" in your request suggests you're probably going to have to break the solution into separate sets of data and then link the two sets in some way. (Your request also needs to be broken into two parts.)

  • "Show me the orders that contain a bike."

Translation

Select the distinct order numbers from the order details table where the product number is in the list of bike product numbers

Clean Up

Select the distinct order numbers from the order details table where the product number is in the list of bike product numbers

SQL

SELECT DISTINCT OrderNumber
FROM Order_Details
WHERE ProductNumber IN (1, 2, 6, 11)

  • "Show me the orders that contain a helmet."

Translation

Select the distinct order numbers from the order details table where the product number is in the list of helmet product numbers

Clean Up

Select the distinct order numbers from the order details table where the product number is in the list of helmet product numbers

SQL

SELECT DISTINCT OrderNumber
FROM Order_Details
WHERE ProductNumber IN (10, 25, 26)

Now you're ready to get the final solution by using—you guessed it—an intersection of the two sets. Figure 7-8 shows the SQL syntax diagram that handles this problem. (Note that you can use INTERSECT more than once to combine multiple SELECT statements.)

Figure 7-8

Figure 7-8 Linking two SELECT statements with INTERSECT

You can now take the two parts of your request and link them with an INTERSECT operator to get the correct answer.

SQL

SELECT DISTINCT OrderNumber
FROM Order_Details
WHERE ProductNumber IN (1, 2, 6, 11)
INTERSECT
SELECT DISTINCT OrderNumber
FROM Order_Details
WHERE ProductNumber IN (10, 25, 26)

The sad news is that not many commercial implementations of SQL yet support the INTERSECT operator. But all is not lost! Remember that the primary key of a table uniquely identifies each row. (You don't have to match on all the fields in a row—just the primary key—to find unique rows that intersect.) We'll show you an alternative method (JOIN) in Chapter 8 that can solve this type of problem in another way. The good news is that most commercial implementations of SQL do support JOIN.

Finding Missing Values: EXCEPT (DIFFERENCE)

Okay, let's go back to the bicycles and helmets problem again. Let's say you're trying to solve this seemingly simple request as follows.

  • "Show me the orders that contain a bike but not a helmet."

Translation

Select the distinct order numbers from the order details table where the product number is in the list of bike product numbers and product number is not in the list of helmet product numbers

Clean Up

Select the distinct order numbers from the order details table where the product number is in the list of bike product numbers and product number is not in the list of helmet product numbers

SQL

SELECT DISTINCT OrderNumber
FROM Order_Details
WHERE ProductNumber IN (1, 2, 6, 11)
AND ProductNumber NOT IN (10, 25, 26)

Unfortunately, the answer shows you orders that contain only a bike! The problem is that the first IN clause finds detail rows containing a bicycle, but the second IN clause simply eliminates helmet rows. If you visualize orders with bicycles and orders with helmets as two distinct sets, you'll find this easier to understand. Figure 7-9 shows one possible relationship between the two sets of orders.

Figure 7-9

Figure 7-9 Orders for a bicycle that do not also contain a helmet

Seeing "except" or "but not" in your request suggests you're probably going to have to break the solution into separate sets of data and then link the two sets in some way. (Your request also needs to be broken into two parts.)

  • "Show me the orders that contain a bike."

Translation

Select the distinct order numbers from the order details table where the product number is in the list of bike product numbers

Clean Up

Select the distinct order numbers from the order details table where the product number is in the list of bike product numbers

SQL

SELECT DISTINCT OrderNumber
FROM Order_Details
WHERE ProductNumber IN (1, 2, 6, 11)

  • "Show me the orders that contain a helmet."

Translation

Select the distinct order numbers from the order details table where the product number is in the list of helmet product numbers

Clean Up

Select the distinct order numbers from the order details table where the product number is in the list of helmet product numbers

SQL

SELECT DISTINCT OrderNumber
FROM Order_Details
WHERE ProductNumber IN (10, 25, 26)

Now you're ready to get the final solution by using—you guessed it—a difference of the two sets. SQL uses the EXCEPT keyword to denote a difference operation. Figure 7-10 shows you the SQL syntax diagram that handles this problem.

Figure 7-10

Figure 7-10 Linking two SELECT statements with EXCEPT

You can now take the two parts of your request and link them with an EXCEPT operator to get the correct answer.

SQL

SELECT DISTINCT OrderNumber
FROM Order_Details
WHERE ProductNumber IN (1, 2, 6, 11)
EXCEPT
SELECT DISTINCT OrderNumber
FROM Order_Details
WHERE ProductNumber IN (10, 25, 26)

Remember from our earlier discussion about the difference operation that the sequence of the sets matters. In this case you're asking for bikes "except" helmets. If you want to find out the opposite case—orders for helmets that do not include bikes—you can turn it around as follows.

SQL

SELECT DISTINCT OrderNumber
FROM Order_Details
WHERE ProductNumber IN (10, 25, 26)
EXCEPT
SELECT DISTINCT OrderNumber
FROM Order_Details
WHERE ProductNumber IN (1, 2, 6, 11)

The sad news is that not many commercial implementations of SQL yet support the EXCEPT operator. Hang on to your helmet! Remember that the primary key of a table uniquely identifies each row. (You don't have to match on all the fields in a row—just the primary key—to find unique rows that are different.) We'll show you an alternative method (OUTER JOIN) in Chapter 9 that can solve this type of problem in another way. The good news is that most commercial implementations of SQL do support OUTER JOIN.

Combining Sets: UNION

One more problem about bicycles and helmets, then we'll pedal on to the next chapter. Let's say you're trying to solve this request, which looks simple enough on the surface.

  • "Show me the orders that contain either a bike or a helmet."

Translation

Select the distinct order numbers from the order details table where the product number is in the list of bike and helmet product numbers

Clean Up

Select the distinct order numbers from the order details table where the product number is in the list of bike and helmet product numbers

SQL

SELECT DISTINCT OrderNumber
FROM Order_Details
WHERE ProductNumber IN (1, 2, 6, 10, 11, 25, 26)

Actually, that works just fine! So why use a UNION to solve this problem? The truth is, you probably would not. However, if we make the problem more complicated, a UNION would be useful.

  • "List the customers who ordered a bicycle together with the vendors who provide bicycles."

Unfortunately, answering this request involves creating a couple of queries using JOIN operations, then using UNION to get the final result. Because we haven't shown you how to do a JOIN yet, we'll save solving this problem for Chapter 10. Gives you something to look forward to, doesn't it?

Let's get back to the "bicycles or helmets" problem and solve it with a UNION. If you visualize orders with bicycles and orders with helmets as two distinct sets, then you'll find it easier to understand the problem. Figure 7-11 shows you one possible relationship between the two sets of orders.

Figure 7-11

Figure 7-11 Orders for bicycles or helmets

Seeing "either," "or," or "together" in your request suggests that you'll need to break the solution into separate sets of data and then link the two sets with a UNION. This particular request can be broken into two parts.

  • "Show me the orders that contain a bike."

Translation

Select the distinct order numbers from the order details table where the product number is in the list of bike product numbers

Clean Up

Select the distinct order numbers from the order details table where the product number is in the list of bike product numbers

SQL

SELECT DISTINCT OrderNumber
FROM Order_Details
WHERE ProductNumber IN (1, 2, 6, 11)

  • "Show me the orders that contain a helmet."

Translation

Select the distinct order numbers from the order details table where the product number is in the list of helmet product numbers

Clean Up

Select the distinct order numbers from the order details table where the product number is in the list of helmet product numbers

SQL

SELECT DISTINCT OrderNumber
FROM Order_Details
WHERE ProductNumber IN (10, 25, 26)

Now you're ready to get the final solution by using—you guessed it—a union of the two sets. Figure 7-12 shows the SQL syntax diagram that handles this problem.

Figure 7-12

Figure 7-12 Linking two SELECT statements with UNION

You can now take the two parts of your request and link them with a UNION operator to get the correct answer.

SQL

SELECT DISTINCT OrderNumber
FROM Order_Details
WHERE ProductNumber IN (1, 2, 6, 11)
UNION
SELECT DISTINCT OrderNumber
FROM Order_Details
WHERE ProductNumber IN (10, 25, 26)

The good news is that most commercial implementations of SQL support the UNION operator. As is perhaps obvious from the examples, a UNION might be doing it the hard way when you want to get an "either-or" result from a single table. UNION is most useful for compiling a list from several similarly structured but different tables. We'll explore UNION in much more detail in Chapter 10.

  • + Share This
  • 🔖 Save To Your Account

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