SQL Server Reference Guide

Mar 28, 2003

␡

⎙ Print

< Back Page 152 of 365 Next >

Last week, we covered the summary statistical algorithms that give additional confidence in the data's credibility. We learned that the most abused statistical measure is the average.

Which brings up a story: it seems that three statisticians were hunting in the woods. A rabbit appeared, and two of them fired their weapons. The first shot far in front of the rabbit and the second shot far behind. "We got him!" exclaimed the third.

While this is a humorous story, it brings up a good point. Without more data, the average isn't the most important statistic.

The last tutorial promised a handy chart to tell which statistical method to use in which situation, and here it is:

Method	Categories	Individual Numbers	Series Numbers
Mode	OK	OK (if multiple values)	Intervals only
Mean	Bad	OK	OK
Median	Ordered sets only	OK	OK

This chart shows the highest confidence in the measure. In most cases, however, the data is mixed. That's why we don't rely on a single value to describe the data.

To finish out the summary functions, here's a code sample to show all the algorithms we've covered so far:

USE pubs
GO
CREATE TABLE #Median(Median int, ID int identity) 
INSERT INTO #Median (Median) 
  SELECT qty 
  FROM sales
  ORDER BY qty ASC
DECLARE @Count AS INT, @MiddleRow AS INT, @Median AS INT 
SET @Count = (SELECT COUNT(*)FROM #Median) 
SET @MiddleRow = (@Count/2) 
SET @Median = (SELECT Median FROM #Median WHERE ID = @MiddleRow)

SELECT 
 COUNT(qty) as 'Count'
, SUM(qty) as 'Sum' 
, AVG(qty) as 'Average'
, @Median as 'Median' 
, STDEV(qty) as 'Std Deviation'
, VAR(qty) as 'Variance'
, MAX(qty) as 'Maximum'
, MIN(qty) as 'Minimum'
FROM sales
DROP TABLE #Median
GO

Representing Data

The next set of algorithms we'll examine are those that represent data. These measures show the type of data we are working with.

We've seen some of these functions and algorithms before, but this time we'll use them to build on our example. The first function we'll examine is COUNT. We covered this example in the last tutorial, so we won't spend a lot of time on it here other than a quick refresher:

SELECT COUNT(qty) as 'Count'
FROM sales
GO

Remember that we can use any of the WHERE constructs we learned to limit the result set.

We've also seen the next measure before, in our tutorial on aggregates, but not named as such: GROUP BY. In a SELECT statement, the GROUP BY clause places the results into a list that shows the number of times a value occurs:

SELECT title_id AS 'Title', COUNT(title_id) as 'Count'
FROM sales
GROUP BY title_id
------------------------------------

Title	Count
BU1032	2
BU1111	1
BU2075	1
BU7832	1
MC2222	1
MC3021	2
PC1035	1
PC8888	1
PS1372	1
PS2091	4
PS2106	1
PS3333	1
PS7777	1
TC3218	1
TC4203	1
TC7777	1

We can also use the ORDER BY clause to order the list on either column:

SELECT title_id AS 'Title', COUNT(title_id) as 'Count'
FROM sales
GROUP BY title_id
ORDER BY 'Count' DESC

This type of output is called a "Frequency distribution," and it is also sometimes found in a chart format. While it's not exactly native to T-SQL to do graphics, it's interesting to see what we can accomplish with a little code:

SELECT title_id AS 'Title'
, REPLICATE('*', COUNT(title_id)) AS 'Count'
FROM sales
GROUP BY title_id
GO
--------------------------------------

Title	Count
BU1032	**
BU1111	*
BU2075	*
BU7832	*
MC2222	*
MC3021	**
PC1035	*
PC8888	*
PS1372	*
PS2091	****
PS2106	*
PS3333	*
PS7777	*
TC3218	*
TC4203	*
TC7777	*

This is the same basic query we saw earlier, but we've added one new construct: REPLICATE. This command merely repeats a character (an asterisk, in this case) by a certain number. To get that number we used the aggregate function of the quantity.

Since most people are visually oriented, this view can be helpful to describe the data set. Of course, this view isn't practical in SQL for large numeric values, unless we break the numbers down into groups of tens or hundreds.

As we can see, this type of data is basically a bar chart on its side. Bar charts are used quite frequently in statistical measures, and once again we have to check them out with a "weather eye." The problem with a bar chart arises with a depiction of scale.

Let's take a look at the scores from the schools in Shelbyville and those in Springfield:

Figure 106

We can see here that the students in Shelbyville had scores around the 98 range, and those in Springfield had scores around 95. Not bad at all, for either city. Not bad, that is, unless we want to show a disparity between the cities, perhaps to increase school funding. In that case, all we have to do is change the scale, starting the bottom value at 94, rather than 0. Take a look at this graph:

Figure 107

Notice that we clearly need more funding in Springfield!

This subject brings us into the next set of descriptive measurements: comparisons. There really isn't a formula or an algorithm for comparing data; what we're after in this measurement is showing two or more data sets side by side. The graphs shown above are examples of that.

The simple joins and unions that we covered earlier are enough for this activity. Nothing complex is called for here. It can also be helpful to group the values to show the disparity between the samples as well:

SELECT stor_id AS 'Store:'
, SUM(qty) AS 'Titles Sold:'
FROM sales
GROUP BY stor_id
---------------------------------------

Store:	Titles Sold:
6380	8
7066	125
7067	90
7131	130
7896	60
8042	80

The only caveat to that is the data should always be "apples to apples," or in a similar sample distribution. The danger here is that the samples could show a disproportionate relationship if they aren't from the same kind of measurement.

One more: here's a script (although it's a bit more complex than I'd like) that provides the percentages for the values:

/* Let's get a aggregated table */
SELECT 
  stor_id AS 'store'
  , SUM(qty) AS quantity
  INTO #testtable
  FROM sales
  GROUP BY stor_id
/* Now let's set aside a decimal variable so the math will work */
DECLARE @var1 AS DECIMAL (5,2)
SET @Var1 = (SELECT sum(quantity)FROM #testtable)
/* Now let's get the values, one line at a 
time, and then divide them by the aggregate */
SELECT store
, quantity
, (quantity/@var1)*100 AS 'Percentage'
FROM #testtable
/* Cleanup */
DROP TABLE #testtable
---------------------------------------

StoreQuantityPercentage 638081.622700 706612525.354900 70679018.255500 713113026.369100 78966012.170300 80428016.227100

Want to extend this a bit further? Just multiply each raw (not multiplied by 100) percentage by 360 to determine the angle for a pie chart! (OK, it might actually be better to do that one in Excel.)

Online Resources

I didn't have a lot of time to spend on the charts in this tutorial, but Matthew Pinkney shows them (along with other interesting math functions) in a better format here.

InformIT Tutorials and Sample Chapters

Need a bit more real-world statistics? Check out Applied Statistics for Software Managers, by Katrina Maxwell.

< Back Page 152 of 365 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