Sams Teach Yourself SQL in 24 Hours

Sep 9, 2005

📄 Contents

␡

⎙ Print

< Back Page 71 of 251 Next >

Recommended Book 

Sams Teach Yourself SQL in 24 Hours, 3rd Edition

Learn More Buy

What Are Aggregate Functions?

Functions are keywords in SQL used to manipulate values within columns for output purposes. A function is a command always used in conjunction with a column name or expression. There are several types of functions in SQL. This hour covers aggregate functions. An aggregate function is used to provide summarization information for an SQL statement, such as counts, totals, and averages.

The aggregate functions discussed in this hour are

COUNT
SUM
MAX
MIN
AVG

The following queries show the data used for most of this hour's examples:

   

   SELECT *

   FROM PRODUCTS_TBL;

   
PROD_ID    PROD_DESC                       COST
---------- ------------------------------ ------
11235      WITCHES COSTUME                29.99
222        PLASTIC PUMPKIN 18 INCH         7.75
13         FALSE PARAFFIN TEETH            1.1
90         LIGHTED LANTERNS               14.5
15         ASSORTED COSTUMES              10
9          CANDY CORN                      1.35
6          PUMPKIN CANDY                   1.45
87         PLASTIC SPIDERS                 1.05
119        ASSORTED MASKS                  4.95
1234       KEY CHAIN                       5.95
2345       OAK BOOKSHELF                  59.99

11 rows selected.

Some employees do not have a pager number in the results of the following query:

   

   SELECT EMP_ID, LAST_NAME, FIRST_NAME, PAGER

   FROM EMPLOYEE_TBL;

   
EMP_ID    LAST_NAM FIRST_NA PAGER
--------- -------- -------- ----------
311549902 STEPHENS TINA
442346889 PLEW     LINDA
213764555 GLASS    BRANDON  3175709980
313782439 GLASS    JACOB    8887345678
220984332 WALLACE  MARIAH
443679012 SPURGEON TIFFANY

6 rows selected.

The `COUNT` Function

The COUNT function is used to count rows or values of a column that do not contain a NULL value. When used with a query, the COUNT function returns a numeric value. When the COUNT function is used with the DISTINCT command, only the distinct rows are counted. ALL (opposite of DISTINCT) is the default; it is not necessary to include ALL in the syntax. Duplicate rows are counted if DISTINCT is not specified. One other option with the COUNT function is to use COUNT with an asterisk. COUNT, when used with an asterisk (COUNT(*)), counts all the rows of a table including duplicates, whether a NULL value is contained in a column or not.

The syntax for the COUNT function is as follows:

   
COUNT [ (*) | (DISTINCT | ALL) ] (COLUMN NAME)

Example	Meaning
`SELECT COUNT(EMPLOYEE_ID) FROM EMPLOYEE_PAY_ID`	Counts all employee IDs
`SELECT COUNT(DISTINCT SALARY)FROM EMPLOYEE_PAY_TBL`	Counts only the distinct rows
`SELECT COUNT(ALL SALARY)FROM EMPLOYEE_PAY_TBL`	Counts all rows for `SALARY`
`SELECT COUNT(*) FROM EMPLOYEE_TBL`	Counts all rows of the `EMPLOYEE` table

COUNT(*) is used in the following example to get a count of all records in the EMPLOYEE_TBL table. There are six employees.

   

   SELECT COUNT(*)

   FROM EMPLOYEE_TBL;

   
COUNT(*)
----------
         6

COUNT(EMP_ID) is used in the next example to get a count of all the employee identifications that exist in the table. The returned count is the same as the last query because all employees have an identification number.

   

   SELECT COUNT(EMP_ID)

   FROM EMPLOYEE_TBL;

   
COUNT(EMP_ID)
-------------
            6

COUNT(PAGER) is used in the following example to get a count of all of the employee records that have a pager number. Only two employees had pager numbers.

   

   SELECT COUNT(PAGER)

   FROM EMPLOYEE_TBL;

   
COUNT(PAGER)
------------
           2

The ORDERS_TBL table, shown next, is used in the following COUNT example:

   

   SELECT *

   FROM ORDERS_TBL;

   
ORD_NUM    CUST_ID    PROD_ID           QTY ORD_DATE_
---------- ---------- ----------------- -------------
56A901     232        11235               1 22-OCT-99
56A917     12         907               100 30-SEP-99
32A132     43         222                25 10-OCT-99
16C17      090        222                 2 17-OCT-99
18D778     287        90                 10 17-OCT-99
23E934     432        13                 20 15-OCT-99
90C461     560        1234                2

7 rows selected.

This last example obtains a count of all distinct product identifications in the ORDERS_TBL table.

   

   SELECT COUNT(DISTINCT(PROD_ID))

   FROM ORDERS_TBL;

   
COUNT(DISTINCT(PROD_ID))
------------------------
                6

The PROD_ID 222 has two entries in the table, thus reducing the distinct values from 7 to 6.

The `SUM` Function

The SUM function is used to return a total on the values of a column for a group of rows. The SUM function can also be used in conjunction with DISTINCT. When SUM is used with DISTINCT, only the distinct rows are totaled, which may not have much purpose. Your total is not accurate in that case because rows of data are omitted.

The syntax for the SUM function is as follows:

   
SUM ([ DISTINCT ] COLUMN NAME)

Example	Meaning
`SELECT SUM(SALARY) FROM EMPLOYEE_PAY_TBL`	Totals the salaries
`SELECT SUM(DISTINCT SALARY) FROM EMPLOYEE_PAY_TBL`	Totals the distinct salaries

In the following query, the sum, or total amount, of all cost values is being retrieved from the PRODUCTS_TBL table:

   

   SELECT SUM(COST)

   FROM PRODUCTS_TBL;

   
SUM(COST)
----------
   163.07

The `AVG` Function

The AVG function is used to find averages for a group of rows. When used with the DISTINCT command, the AVG function returns the average of the distinct rows. The syntax for the AVG function is as follows:

   
AVG ([ DISTINCT ] COLUMN NAME)

Example	Meaning
`SELECT AVG(SALARY) FROM EMPLOYEE_PAY_TBL`	Returns the average salary
`SELECT AVG(DISTINCT SALARY) EMPLOYEE_PAY_TBL`	Returns the distinct `FROM` average salary

The average value for all values in the PRODUCTS_TBL table's COST column is being retrieved in the following example:

   

   SELECT AVG(COST)

   FROM PRODUCTS_TBL;

   
AVG(COST)
----------
13.5891667

The next example uses two aggregate functions in the same query. Because some employees are paid hourly and others paid a salary, you want to retrieve the average value for both PAY_RATE and SALARY.

   

   SELECT AVG(PAY_RATE), AVG(SALARY)

   FROM EMPLOYEE_PAY_TBL;

   
AVG(PAY_RATE) AVG(SALARY)
------------- -----------
   13.5833333       30000

The `MAX` Function

The MAX function is used to return the maximum value for the values of a column in a group of rows. NULL values are ignored when using the MAX function. The DISTINCT command is an option. However, because the maximum value for all the rows is the same as the distinct maximum value, DISTINCT is useless.

   
MAX([ DISTINCT ] COLUMN NAME)

Example	Meaning
`SELECT MAX(SALARY) FROM EMPLOYEE_PAY_TBL`	Returns the highest salary
`SELECT MAX(DISTINCT SALARY) FROM EMPLOYEE_PAY_TBL`	Returns the highest distinct salary

The following example returns the maximum value for the COST column in the PRODUCTS_TBL table:

   

   SELECT MAX(COST)

   FROM PRODUCTS_TBL;

   
MAX(COST)
----------
    59.99

The `MIN` Function

The MIN function returns the minimum value of a column for a group of rows. NULL values are ignored when using the MIN function. The DISTINCT command is an option. However, because the minimum value for all rows is the same as the minimum value for distinct rows, DISTINCT is useless.

   
MIN([ DISTINCT ] COLUMN NAME)

Example	Meaning
`SELECT MIN(SALARY) FROM EMPLOYEE_PAY_TBL`	Returns the lowest salary
`SELECT MIN(DISTINCT SALARY) FROM` `EMPLOYEE_PAY_TBL`	Returns the lowest distinct salary

The following example returns the minimum value for the COST column in the PRODUCTS_TBL table:

   

   SELECT MIN(COST)

   FROM PRODUCTS_TBL;

   
MIN(COST)
----------
     1.05

The final example combines aggregate functions with the use of arithmetic operators:

   

   SELECT COUNT(ORD_NUM), SUM(QTY),
       
   SUM(QTY) / COUNT(ORD_NUM) AVG_QTY

   FROM ORDERS_TBL;

   
COUNT(ORD_NUM)   SUM(QTY)    AVG_QTY
-------------- ---------- ----------
             7        160 22.857143

You have performed a count on all order numbers, figured the sum of all quantities ordered, and, by dividing the two figures, have derived the average quantity of an item per order. You also created a column alias for the computation—AVG_QTY.

< Back Page 71 of 251 Next >

🔖 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.

Privacy Notice

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

Email Address