Home > Articles > Data > SQL Server

This chapter is from the book

Difference

What's the difference between 21 and 10? If you answered 11, you're on the right track! A difference operation (sometimes also called subtract, minus, or except) takes one set of values and removes the set of values from a second set. What remains is the set of values in the first set that are not in the second set. (As you'll see later, EXCEPT is the keyword used in the SQL Standard.)

Difference in Set Theory

Difference is another very powerful mathematical tool. As a scientist, you might be interested in finding what's different about two sets of chemical or physical sample data. For example, a pharmaceutical research chemist might have two compounds that seem to be very similar, but one provides a certain beneficial effect and the other does not. Finding what's different about the two compounds might help uncover why one works and the other does not. As an engineer, you might have two similar designs, but one works better than the other. Finding the difference between the two designs could be crucial to eliminating structural flaws in future buildings.

Let's take a look at difference 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 difference of the first set of numbers minus the second set of numbers is the numbers that exist in the first set but not the second.

  • 1, 5, 9, 32, 78

Note that you can turn the previous difference operation around. Thus, the difference of the second set minus the first set is

  • 3, 7, 22, 71, 99

The members of each set don't have to be single values. In fact, you'll most likely be dealing with sets of rows when trying to solve problems with SQL.

Earlier in this chapter we said that when a member of a set is something more than a single number or value, each member of the set has multiple attributes (bits of information that describe the properties of each member). For example, your favorite stew recipe is a complex member of the set of all recipes that contains many different ingredients. You can think of each ingredient as an attribute of your complex stew member.

To find the difference between two sets of complex set members, you have to find the members that match on all the attributes in the second set with members in the first set. Don't forget that all of the members in each set you're trying to compare must have the same number and type of attributes. Remove from the first set all the matching members you find in the second set, and the result is the difference. 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 this.

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 difference of the first set minus the second set is the objects in the first set that don't exist in the second set.

Potatoes

Water

Lamb

Peas

Rice

Chicken Stock

Chicken

Carrots

Pasta

Water

Tofu

Snap Peas

Pasta

Water

Pork

Onions

Difference between Result Sets

When you're dealing with rows in a set of data fetched 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 the following. (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 difference between John's recipes and Mike's recipes (John's minus Mike's) is all the recipes in John's cookbook that do not appear in Mike's cookbook.

Recipe

Starch

Stock

Meat

Vegetable

Chicken Stew

Rice

Chicken Stock

Chicken

Carrots

Veggie Stew

Pasta

Water

Tofu

Snap Peas

Pork Stew

Pasta

Water

Pork

Onions

You can also turn this problem around. Suppose you want to find the recipes in Mike's cookbook that are not in John's cookbook. Here's the answer.

Recipe

Starch

Stock

Meat

Vegetable

Turkey Stew

Rice

Chicken Stock

Turkey

Carrots

Veggie Stew

Pasta

Vegetable Stock

Tofu

Snap Peas

Pork Stew

Beans

Water

Pork

Onions

Again, we can use a set diagram to help visualize how a difference operation works. Let's assume you have a nice database containing all your favorite recipes. You really do not like the way onions taste with beef, so you're interested in finding all recipes that contain beef but not onions. Figure 7-3 shows you the set diagram that helps you visualize how to solve this problem.

Figure 7-3

Figure 7-3 Finding out which recipes have beef but not onions

The upper full circle represents the set of recipes that contain beef. The lower full circle represents the set of recipes that contain onions. As you remember from the discussion about INTERSECT, where the two circles overlap is where you'll find the recipes that contain both. The dark-shaded part of the upper circle that's not part of the overlapping area represents the set of recipes that contain beef but do not contain onions. Likewise, the part of the lower circle that's not part of the overlapping area represents the set of recipes that contain onions but do not contain beef.

You probably know that you first ask SQL to fetch all the recipes that have beef. Next, you ask SQL to fetch all the recipes that have onions. (As you'll see later in this chapter, the special SQL keyword EXCEPT links the two queries to get the final answer.)

Are you falling into the trap again? (You did read Chapter 2, didn't you?) If your recipe table looks like the samples earlier, you might think that you could simply say the following.

  • "Show me the recipes that have beef as the meat ingredient and that do not have onions as the vegetable ingredient."

Translation

Select the recipe name from the recipes table where meat ingredient is beef and vegetable ingredient is not onions

Clean Up

Select the recipe name from the recipes table where meat ingredient is = beef and vegetable ingredient is not <> onions

SQL

SELECT RecipeName
FROM Recipes
WHERE MeatIngredient = 'Beef'
AND VegetableIngredient <> 'Onions'

Again, as you learned in Chapter 2, a single Recipes table isn't such a hot idea. (Pun intended!) What about recipes that have ingredients other than meat and vegetables? What about the fact that some recipes have many ingredients and others have only a few? 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 and onions at the same time. You'll need to first find all the beef rows, then find all the onions rows, then difference them on RecipeID.

How about a more complex problem? Let's say you hate carrots, too. A set diagram to visualize the solution might look like Figure 7-4.

Figure 7-4

Figure 7-4 Finding out which recipes have beef but no onions or carrots

First you need to find the set of recipes that have beef, and then get the difference with either the set of recipes containing onions or the set containing carrots. Take that result and get the difference again with the remaining set (onions or carrots) to leave only the recipes that have beef but no carrots or onions (the light-shaded area in the upper circle).

Problems You Can Solve with Difference

Unlike intersection (which looks for common members of two sets), difference looks for members that are in one set but not in another set. Here's just a small sample of the problems you can solve using a difference technique with data from the sample databases.

  • "Show me customers whose names are not the same as any employee."
  • "Find all the customers who ordered a bicycle but did not order a helmet."
  • "List the entertainers who played engagements for customer Bonnicksen but did not play any engagement for customer Rosales."
  • "Show me the students who have an average score of 85 or better in Art but do not 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 but not at Bolero Lanes."
  • "Show me the recipes that have beef but not garlic."

One of the limitations of using a pure difference is that the values must match in all the columns in each result set. This works well if you're finding the difference between 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 finding the difference between sets from tables that have similar columns—for example, customer names and employee names.

In many cases, however, you'll want to find solutions that require a match on only a few column values from each set. For this type of problem, SQL provides an OUTER JOIN operation, which is an intersection on key values that includes the unmatched values from one or both of the two sets. Here's a sample of problems you can solve with an OUTER JOIN.

  • "Show me customers who do not live in the same city as any employees." (OUTER JOIN on the city name.)
  • "List customers and the entertainers they did not book." (OUTER JOIN on the engagement number.)
  • "Find the agents who are not in the same ZIP Code as any entertainer." (OUTER JOIN on the ZIP Code.)
  • "Show me the students who do not have the same first name as any teachers." (OUTER JOIN on the first name.)
  • "Find the bowlers who have an average of 150 or higher who have never bowled a game below 125." (OUTER JOIN on the bowler ID from two different tables.)
  • "Display all the ingredients for recipes that do not have carrots." (OUTER JOIN on the recipe ID.)

Don't worry! We'll show you all about solving these problems (and more) using OUTER JOINs in Chapter 9. Also, because few commercial implementations of SQL support EXCEPT (the keyword for difference), we'll show how to use an OUTER JOIN to solve many problems that might otherwise require an EXCEPT.

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