SQL Server Reference Guide

Mar 28, 2003

␡

⎙ Print

< Back Page 160 of 365 Next >

Today I'll bring some of the Design Elements concepts together and cover program flow. Strictly speaking, program flow is the control over the direction the parts of the programs take based on loops, conditions, and other branching statements. I'm going to broaden the topic a bit to include some simple diagram techniques. Along the way, I'll introduce a couple of branching statements that I haven't covered yet.

Let's begin by creating a simple program requirement. First, we'll review the branching and block syntax we have available, then I'll show you some primitive diagramming for the program flow, and finally I'll implement that flow in a small section of code.

In the tutorial on An Outline for Development, I explained the phases for creating a program:

Understand the goal or problem as completely as possible
Comment the process
Code the comments
Optimize the code

As part of the first phase, it's important to get the system's requirements from the users, and communicating the design for the program to the developers. This is a necessary step, even if that developer is you. You can also use this design to show your understanding of the process to the users. >From the design you create, you can move on to the second phase of the outline above where you comment the code.

Some people work well from textual descriptions. Most of us, however, work better from a graphical representation. We see patterns that aren't easily explained with text alone. It's not difficult; basically, all you need are a few symbols that indicate the flow of the process and some text to explain the symbols.

Several tools have evolved over the years to diagram program flow. These systems range in complexity from three simple shapes (circle, square, and diamond) all the way to a full graphical vocabulary like you see in Universal Markup Language (UML). In this tutorial, I'll use fairly primitive symbols. I'll do that since the example I'll show doesn't require more than that, and I feel that the fewer the symbols the better. In other words, you should use the minimum number of symbols necessary to make the meaning clear.

Before we begin, let's review the syntax SQL provides for program flow, and how it affects programming in T-SQL. I've covered most of these constructs in other articles, and I promised to show you when they all come together. It is here in Program Flow where that starts to happen.

To begin, we have (appropriately enough) the BEGIN...END statements. Anything between these words is treated as a code block, and you can provide error trapping and other branching inside a block.

The WHILE statement creates a condition for a loop. WHILE causes the system to repeat the code block until the condition is met, or until a BREAK statement is encountered. If needed, the CONTINUE statement can restart the BREAK that is created. To leave a block no matter what a result is, you can use the RETURN statement.

Similar to the WHILE statement is the IF...ELSE statement. The IF statement tests a condition and then carries out the program statements that follow, or runs the code behind the ELSE statement.

To branch off to another section of code, you can use the GOTO statement, along with a Label. To create a label, you simply type a single word (or join two or more words with underlines) and end it with a colon, like this:

My_Label:

A new statement that I haven't covered yet is the CASE statement. This statement allows you to create one SELECT, and get different responses based on a test. Here's the syntax:

SELECT 'SomeTextHere' =
CASE
 WHEN fieldname IS NULL THEN 'StringYouWant'
 WHEN fieldname = 'VariableOrValue' THEN 'OtherStringYouWant'
 WHEN fieldname < 'VariableOrValue' THEN 'OtherStringYouWant'
 WHEN fieldname > 'VariableOrValue' THEN 'OtherStringYouWant'
 WHEN fieldname <> 'VariableOrValue' THEN 'OtherStringYouWant'
 ELSE 'WhateverCatchesTheRest' 
END

You would replace the fieldname with a field you're testing against. You can set the return value of the CASE statement with a string (as I have here) or a constant, even a variable. Each one can be different. Just remember to close it all out with an END. We'll see this in our example code a little later.

Finally, there's the WAITFOR statement. I haven't covered this statement until now, so let's take a moment and see it in action.

The WAITFOR statement is clock-based. You can specify an absolute time like this:

USE master
/* Backs up the pubs database at 10:00 in the morning */
BEGIN
WAITFOR TIME '10:00'
BACKUP DATABASE pubs 
	TO DISK = 'c:\temp\pubs.bak' 
END

Or you can have the command run relatively to the current time, like this:

USE master
/* Backs up the pubs database in one minute */
BEGIN
WAITFOR DELAY '00:01:00'
BACKUP DATABASE pubs 
	TO DISK = 'c:\temp\pubs.bak' 
END

These constructs are the tools we have to work with so far, now let's explore the graphical tools that reference them.

As I mentioned, we'll use fairly simple graphical components to illustrate the program flow. You can use specialized tools that create flowcharts, or you can use something as simple as a graphics program to create the model, since it only contains three shapes and one arrow type. Many Microsoft-centric shops use Visio, but the shapes I'm showing here were created with Open Office's presentation package.

There are only three shapes I'll use: a circle, a box, and a diamond. I'll show the program direction with arrows.

In this simple schema, one circle represents the start and another represents the end of the process. A square represents a step, and the diamond represents a choice or branching operation.

In our fictional program, the requirements have been presented to us as follows:

"We need a stored procedure written that accepts an author's name and a book title as an input. The stored procedure will determine if the author wrote that book or another, or no books at all. If the author wrote the book passed as a variable, then the stored procedure should return the word 'Yes' as the answer, and if that author wrote another book that title should be returned. If the name provided isn't in the database, the words 'Not on File' should be returned."

I explained how to break this kind of request down into proper requirements in the article called Database Design: Requirements, Entities, and Attributes. In this exercise, we'll assume that's been done. I'll create a flow diagram of how the stored procedure might be written. You might wonder if I really take the time to flow-chart something this trivial (or whether it even needs a stored procedure at all!). The answer is... well, sometimes. Most of the time, you wouldn't need a full diagram for something as simple as this, but if this request is part of a larger design, you certainly would. For this exercise, a simple diagram is useful to illustrate the process of creating a graphical representation of the stored procedure.

Let's begin with a circle. Since it's obvious that the stored procedure has a start point, I use the start point to indicate any input parameters that are required.

Figure 128

Notice that I've also included the type of input that I'll receive from the calling program. You don't have to provide this information here, but it certainly can help in the flow diagram since a string of characters has more branching operations than a bit data type.

You can show as much granularity as you like. I normally include the SELECT statement operation, but you could go directly to the decision element.

Figuree 129

Once I locate the author's name (if it exists) I have a decision to make. Based on the results of the query, there are three possible outcomes. One is that the author wrote that title, in which case I'll return the string "Yes". If the author wrote another book, I'll have to find out what the title is and then provide that title as the return string. Finally, if the author doesn't exist, I'll return the string "Not on File" to the calling program. Here's what I came up with:

Figure 130

Notice that the diamond contains a question. The answers are shown to the right, and the circles at the end of each decision contain the strings that are returned to the calling program based on the decision.

If the results of the decision create a situation wherein more steps are needed, a combination of boxes would illustrate those steps. Eventually, the steps culminate in a return or end value, or re-join another step or decision. Using arrows, the developer can trace what results are needed based on the results he or she gets from their queries.

What we're really doing here is abstracting the process so that both the developer and the business user can see the logic. This helps you avoid confusion and frustration on everyone's part.

Based on what I have here, here's one possible stored procedure:

USE pubs
GO

CREATE PROCEDURE FindAuthor
 @firstname varchar(18)
, @lastname varchar(30)
, @titlename varchar(100)

AS 

SELECT 'TitleName' =
CASE
 WHEN t.title IS NULL THEN 'Not on File'
 WHEN t.title = @titlename THEN 'Yes'
 ELSE title
END

FROM authors a 
 INNER JOIN titleauthor ta
 	ON a.au_id = ta.au_id 
  INNER JOIN titles t
	ON t.title_id = ta.title_id 

WHERE au_fname LIKE @firstname
 AND au_lname LIKE @lastname

You can see the results with this statement:

EXEC FindAuthor 'Johnson', 'White', 'Prolonged Data Deprivation: Four Case Studies'

There's one fatal flaw in the original requirement. It seems that an author can write more than one book. How could we handle that? We'll leave that discussion to another time.

Online Resources

Want to learn more about UML? Paul Evitts has a great introduction to the elements it contains.

InformIT Tutorials and Sample Chapters

The seminal work on design patterns is still the best: http://www.informit.com/title/0201633612

< Back Page 160 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