CCAPPENDIX

WEB PROGRAMMINGThe Web was originally designed to deliver static Web pages from a Web serverconnected somewhere on the Internet to a Web browser sitting on a user’s desktopcomputer. Basically, all a user could do was click on a hot spot or hypertext link toretrieve a new page, read it, and then go on to the next page. The Web was notdesigned to support EC sites, especially B2C sites. In its original state, it was notpossible to create pages that would allow consumers to easily determine what prod-ucts were for sale, to select products as they moved from page to page (i.e., an elec-tronic shopping cart), to place an order, or to verify an order. Similarly, there was nosimple way to integrate a Web server with a database system containing product,pricing, and promotional data with transactional systems for processing orders andwith payment systems for handling credit card purchases and settlements.

Over time, these limitations have been addressed. First, forms were added toHTML. Forms provided a way to produce Web pages from which a consumer couldselect, order, and pay for products. Second, special programming and scripting lan-guages (e.g., Java and JavaScript) were created. These newer languages allowedapplication developers to produce interactive Web pages whose functionality emu-lated the rich functionality of standard Windows-based applications. Finally, a stan-dard application programming interface (API), called the common gateway inter-face (CGI), was introduced. Generally speaking, an API provides a way for onesoftware program to communicate with another, whereas CGI provides a way forsoftware developers and application programmers to integrate Web servers with var-ious back-end programs and data sources. Because of CGI’s inefficiencies, newerAPIs and special database gateway programs were also introduced. As a result ofthese changes, the Web is now well suited for the dynamic world of EC.

This appendix examines issues of end-user interactivity and dynamic dataaccess. The first sections focus on Java and JavaScript, which are special program-ming languages that can be used to create Web pages with rich graphical userinterfaces (GUIs). The remaining sections examine various methods—CGI pro-gramming, specialized APIs, and server-side scripting—for integrating a Webserver with back-end programs, including relational databases.

DESKTOP INTERACTIVITY AND PROCESSINGMost of us are familiar with various applications running on Microsoft Windows.One of the hallmarks of a Windows application is its GUI (i.e., its look and feel)and the on-screen interactivity it provides. To regular Windows users, scrolling,clicking, double-clicking, dragging and dropping, entering data, and similaractions are almost second nature. In Windows, the desktop computer on whichthe application resides handles these actions. For instance, if a user of MicrosoftExcel enters a formula and hits the “Enter” key, then the desktop computer calculatesthe result, which is displayed immediately in the Excel spreadsheet. Even with a

application program-ming interface (API)A standard that providesa way for one softwareprogram to communi-cate with another.

common gatewayinterface (CGI)A standard that providesa way for softwaredevelopers and applica-tion programmers tointegrate Web serverswith various back-endprograms and datasources.

C-2 Appendix C

client/server application where processing is distributed between a desktop com-puter and a back-end server, the desktop still handles the user’s actions.

Initially, the type of interactivity found in a Windows application was missingfrom the Web. The earliest Web pages were built solely with HTML (described indetail in Appendix B). Until forms were added to HTML, the only action sup-ported by HTML was the clicking on a hypertext link to access a new page. Formsadded data entry capabilities and a series of new elements (e.g., radio buttons, checkboxes, and dropdown selection lists). However, even with these new features, theonly thing a user could do was click. In the same vein, there was no way for thebrowser (i.e., the user’s desktop) to process the user’s actions. All the processing hadto occur on a Web server. Although this is enough interactivity for some applica-tions, many applications are better served by the types of interactions supported byWindows. This is where Java and scripting languages such as JavaScript come intoplay. Java and JavaScript provide the means to create applications that support thetypes of user interactions found in Windows. They also make it possible for pro-cessing to be distributed between the desktop and the server.

JAVAJava’s roots are in work done at the beginning of the 1990s by a developer namedJames Gosling at Sun Microsystems. This original work focused on developingsoftware and networks for consumer devices (such as VCRs, PDAs, toasters, and soforth). The result of this work was a programming language called Oak. Recogniz-ing the growing importance of the Internet, the developers at Sun Microsystemsturned their attention from consumer devices to making Oak a premier program-ming language for the Web. In 1995, Oak was renamed Java. In that same year, anumber of software and hardware vendors licensed Java from Sun, including IBM,Netscape, Microsoft, and Symantec. Today, it is estimated that there are over 1 mil-lion programmers using Java to develop Web and network-based applications.

Java is similar to C�� and can be used to produce stand-alone applications orapplets. An applet is a Java program that is written for and runs in a Web browser.As a programming language, Java is well suited to Web development because of itsnative support for Internet communications.

If Java was simply a programming language, then it would not have generatedmuch interest outside the programming community. After all, how many of us areexcited or know much about C��, even though it is the development languageused in most commercial software products? What makes Java interesting to indus-try observers is that it also has a run-time environment called a virtual machine(VM). Any computer that has a Java VM can run a Java program. All Web browsershave built-in Java VMs, although in the future users of the Microsoft browser(Internet Explorer 6.0) will have to download a Java VM to their desktops.

Java differs from other programming language in one important respect. Otherprogramming languages require developers to write and then compile their pro-grams on the machine on which the program is to be run. Compilation refers to theprocess of converting the human readable form of a program into binary ormachine code that can be run by a specific computer. For example, if a developerwants a program to run on a PC and a Unix computer, then the developer must

virtual machine (VM)A run-time environmentin Java that enables anycomputer to run a Javaprogram; all Webbrowsers have built-inJava VMs.

Appendix C C-3

write and compile the program on both machines. Even with the same type ofcomputer (e.g., a PC), the program must be written for and compiled on each gen-eration of the machine or the machine’s operating system (e.g., Windows 98 versusWindows 2000). Of course, you can see that standard program languages are illsuited for the Web because of the large variety of computers that are attached to it.In contrast, Java is a “write once, run anywhere” (WORA) programming language.That is, a programmer writes a Java program and compiles it. Unlike other languagecompilers that turn a program into machine-specific code, the Java compiler pro-duces an intermediate form of code (called byte code) that is not specific to a particu-lar machine. When Java byte code is sent to a Web browser, the Java VM executesit on the fly. Where does the Java VM reside? It comes with the Web browser.

Special <APPLET> tags are used to incorporate applets within an HTMLpage. Exhibit C.1 shows a Java-based spreadsheet and chart that might be used toanalyze sales data for online beverage, food, and nonconsumable purchases. Thisparticular page has two Java applets—a grid and a chart. Both of these appletsallow the end user to navigate through the data, selecting different slices of data,and to drill down into underlying detail. Although the data might be displayed ina static HTML table and image (a .jpg or .gif file), a static table or image could notprovide end users with the same interactive capabilities. The code behind part ofthis page is shown in Exhibit C.2. The applets are defined by the <APPLET> . . .

EXHIBIT C.1 Java Spreadsheet from Comshare Decision

Source: Comshare Decision, 2002.

C-4 Appendix C

</APPLET> tags. With each of these applets, the “CODE” parameter indicatesthe Java code to be downloaded from the Web server. The WIDTH andHEIGHT parameters of the <APPLET> tags define rectangular regions for dis-playing the applets on the Web page.

When a Web browser accesses a page with an <APPLET> tag, it begins by render-ing the page. During this process, it sees the <APPLET> tag and knows to (1) reservea display space within the page and (2) request the Web server to send the Java appletcode. Once the applet arrives, the browser’s Java VM begins executing the applet code.

The Java VM handles any interaction that takes place between the user and anapplet. It is as if there is a separate program running within the page. One advan-tage of having the Java VM in control is that it can enforce security, providing a“sandbox” that limits the applet’s access to system resources. For the most part,applets do not have access to any system resources. For instance, applets cannotread from or write to files on the end user’s computer. This is why it is difficult fora hacker to write an applet that plays havoc with a user’s system. However, thereare drawbacks to this restriction. The fact that an applet cannot read or write todisk means that an end user cannot store interim work or the results of any analy-sis. This sort of functionality is crucial for those users (e.g., mobile executives) whodo not have constant access to the Web server.

In spite of its power, few EC applications use Java applets. Most applicationsstill rely on Web forms and client-side scripting (see discussion later in appendix)to provide end-user interactivity. However, Java is used quite extensively for

<HTML><HEAD><TITLE>DecisionWeb</TITLE></HEAD><BODY><P><APPLET CODE�”CDGrid.class” CODEBASE�”/sales/classes”WIDTH�700 HEIGHT�250name�Grid><PARAM name�ViewName value�”SalesData”></APPLET></P><P><APPLET CODE�”CDChart.class” CODEBASE�”/sales/classes”WIDTH�700 HEIGHT�250name�Chart><PARAM name�ViewName value�”SalesData”></APPLET></P></BODY></HTML>

EXHIBIT C.2 Java Applet Tags for Spreadsheet in Exhibit C.1

Appendix C C-5

server-side processing in the form of Java beans and Java servlets. Over the pastyear, Microsoft has introduced a Java competitor called C# (i.e., “C sharp”). UnlikeJava, C# runs only on computers with a Microsoft operating system (i.e., Win-dows XP, Windows 2000, etc.).

JAVASCRIPT AND THE DOCUMENT OBJECT MODELJava applets are not the only way to add interactivity to Web pages. Interactivity canalso be provided through client-side scripting using a language such as JavaScript.JavaScript, often confused with Java, is not a lightweight version of Java. The twolanguages have different capabilities and are used for different purposes. JavaScriptis used to control the objects, content, and interactions within a Web page. UnlikeJava, JavaScript cannot be used to produce stand-alone applications.

To program in JavaScript, you simply write the JavaScript programming state-ments and functions directly in the Web page, interspersed with the HTML state-ments. When a Web browser receives a Web page containing JavaScript, it auto-matically knows to execute the program. Exhibit C.3 shows how JavaScript isembedded among the HTML statements on a Web page. The page shown inExhibit C.4 is what results when the code is executed.

In Exhibit C.3, the JavaScript program is contained between the<SCRIPT> . . . </SCRIPT> tags. Without going into the details, when the userclicks the SUBMIT button, the function “onClick” is executed. The functionchecks to see if the value of the “Name” input field in this particular form is empty.

JavaScriptA language, distinctfrom Java, that is usedto control the objects,content, and interactionswithin a Web page;unlike Java, it cannotbe used to producestand-alone applications.

<HTML><HEAD><TITLE>E-Commerce</TITLE></HEAD><SCRIPT LANGUAGE � “javascript”>

FUNCTION valcheck(){if (document.forms[0].elements[0].value �� ““){alert(“Please enter a name”);}}</SCRIPT><BODY BGCOLOR � “#FFFFFF”><H2>Javascript Validation</H2>

<FORM METHOD � POST<P>Name: <INPUT TYPE � “text” NAME � “T1” SIZE � “32”><P><INPUT TYPE � “button” NAME � “B1” VALUE � “Submit” onClick � “valcheck()”>

</FORM></BODY></HTML>

EXHIBIT C.3 JavaScript Validating User Input

C-6 Appendix C

EXHIBIT C.4 Java Form with Validated Input

If it is empty, then an alert window pops up the message, “Please enter a username.” Otherwise, the form is submitted.

When JavaScript was first introduced, one of its major purposes was to vali-date user input (just like the example in Exhibit C.4). Today, it is used extensivelyat EC sites to enhance the dynamic character of Web pages and to accomplish thesorts of tasks listed in Exhibit C.5 (on p. C-7).

Like everything else on the Web, JavaScript has gone through a number ofchanges. In the past couple of years, JavaScript has been intimately tied to the setof elements that can be contained in a Web page. These elements include the textand all the other components of a page, such as HTML tables, images, buttons,forms, and so forth. The collection of possible Web page elements is called thedocument object model (DOM). For each of these elements in the DOM, thereis a well-defined set of user actions that the element can recognize—in program-ming vernacular, these actions are called events. Events include things such asmouse actions, keyboard entry, and changes in the state of a document (e.g., load-ing a document is a state). The collection of events is called an event model. Takentogether, the DOM and event model determine how a scripting language such asJavaScript can be used to manipulate a Web page. Today, the combination of JavaScript and the DOM provide a rich environment for creating Web appli-cations that closely mirror the types of interactivity provided by stand-aloneWindows applications. For a good introduction to JavaScript, see Harris (2001) orNegrino and Smith (2001).

document objectmodel (DOM)In JavaScript, the col-lection of possible Webpage elements, for each of which there is a well-defined set ofuser actions (events)that the element canrecognize.

Appendix C C-7

EXHIBIT C.5 Functions of JavaScriptFunction ExampleClient-side form validation Ensuring that there are no blank input fields, that numbers

are numbers and text fields are text, that the length of afield (like a ticker) is the correct length, and that numericinput values fall within a given range

Client-side calculations As users input or select specific values, new calculationscan be computed and immediately displayed on the page.Examples include mortgage calculations, currencyconversions, and calculating the price of a product basedon the features selected by a user.

Client-side lookup databases JavaScript provides associative data arrays that enablevalues to be looked up on the basis of keys. For instance,given a company name, an associate array could providethe stock ticker (e.g., StockTicker(“Comshare”)�”CSRE”). Simple databases can be embedded in JavaScript, providing lookup tables that can assist end users with data input.

Client-side image maps Image maps are large graphics that are divided into areaswhere each area provides a different response whenclicked by an end user. For instance, an image map mightrepresent the U.S. states. When the user clicks on aparticular state, a specified Web page is retrieved. In aclient-side image map, each of the areas can be associatedwith particular JavaScript functions, making it possible to carry out a variety of client-side actions besides simply retrieving a page.

Client-side interaction The combination of JavaScript and the browser DOMprovides the means to create user interfaces that mirrorstandard Microsoft Windows interfaces that respond toevents such as drag and drop, etc.

Personalizing a document If the JavaScript code is contained in the <HEAD> of abefore it is displayed document, the code is executed before the document is

displayed. In this way personalized documents can becreated based on the date, data contained in cookies, etc.

Manipulating Java applets JavaScript can be used along with various input objectsor ActiveX objects (e.g., buttons) to control applets or ActiveX objectswithin a page contained on the same page. For instance, there might be

an applet displaying a graph and a series of buttons linkedto script that enable a user to select the type of graph,background color, etc.

C-8 Appendix C

A WORD ABOUT ACTIVEX AND VBSCRIPTWhen Java and JavaScript first appeared, Microsoft saw them as a direct threat to itscontrol of the desktop marketplace. With these languages, software vendors could cre-ate desktop and front-office applications (word processing, spreadsheets, and the like)that could be stored on any Web server (not just Microsoft’s Internet InformationServer, IIS), downloaded to any Web browser (not just Microsoft’s Internet Explorer),and run on any computer (not just a PC with Microsoft Windows or NT). Clearly,widespread adoption of Java and JavaScript applications could have reduced Microsoft’smarket share (especially on the desktop), although this has not come to pass. One of thethings that Microsoft did in response to this potential threat was to introduce its ownproprietary components and scripting language called ActiveX components andVBScript (Visual Basic Script), respectively. Although both are designed for the Web,they work only with Microsoft’s Internet Explorer and Microsoft’s Web server.

Like Java applets, ActiveX components are stored on a Web server. When auser of Internet Explorer downloads a Web page referencing an ActiveX compo-nent, the browser retrieves the component and executes it. Unlike Java, ActiveXdoes not run in a “sandbox.” ActiveX components can access system resources(e.g., reading from and writing to files on the desktop).

All of Microsoft’s office applications—Word, Excel, Access, PowerPoint, andothers—come with a scripting language called Visual Basic for Applications(VBA), which can be used to write everything from simple macros for automatingvarious manual processes to full-blown custom applications built on top of one ofthe desktop applications. You can think of VBScript as VBA for Internet Explorer.VBScript works the same way as JavaScript and has the same capabilities. Thismeans that VBScript programs, intertwined with the HTML on a Web page, cancontrol the elements of the DOM and their associated events. In fact, bothVBScript and JavaScript can be included on the same Web page. Of course, itrequires Internet Explorer to run the page. If the page were accessed withNetscape’s Navigator, the VBScript would be ignored and the JavaScript wouldrun, even though the page would not make much sense.

DELIVERING DYNAMIC CONTENTMost EC sites on the Web provide some level of personalization, interactivity, anddata access. In this case, static pages that are constructed ahead of time and storedon a server just will not work. With a static page every user sees the same content.Imagine, for instance, a Web site such as Yahoo! that provides stock quotes that areupdated every 15 minutes. There is no practical way for a site like this to “grab” theprice/volume data for every stock and then generate new static pages (includingtables and charts) every 15 minutes. Obviously, other external programs runningon the Web server (or some other server) are used to generate “dynamic” Webpages on the fly in response to individual user requests. In this case, when an indi-vidual user requests an update on a particular stock (say, Microsoft, whose ticker isMSFT), the external program receives the request, grabs the new stock quote (inreal time), and then generates a Web page with the updated information. How thisis actually accomplished is detailed in the following sections.

Appendix C C-9

CGI PROGRAMMINGOriginally, Web pages with dynamic content were generated with the aid of a CGIprogram. How CGI works is diagrammed in Exhibit C.6. From a technical stand-point, CGI is a set of standard methods and routines used to write a stand-alonesoftware program that knows how to receive requests from a Web server andreturn data to the server. In other words, CGI is a standard protocol used to con-verse with a Web server. A program that has this capability is called a CGI pro-gram. These programs can be written in a wide variety of programming languages.On Unix, the Perl programming language is often used. On Microsoft Windows,Visual Basic or C/C�� are frequently used.

To illustrate the basics of CGI programming, consider the simple exampleshown in Exhibit C.7. In this example, the user can obtain the current stock price(within the last 15 minutes) for either Microsoft or Oracle by clicking on the com-pany name. Behind each name is a URL (see the code in Exhibit C.8). In this case,the URL refers to a program rather than a static document. For instance, the URLassociated with the Microsoft selection is “www.stocknews.com/cgi-bin/ find-stockprice.exe?ticker�MSFT.” Based on this URL, the Web server that receivesthe request is “www.stocknews.com.”

Upon receipt, the server knows that it needs to invoke a CGI program called“stock.exe” rather than retrieve a particular Web document. When it invokes theprogram, it passes the program the information after the “?” mark. The infor-mation after the “?” mark is called a query string. The query string specifies vari-ous parameters and values that the CGI program will use during execution. In this case, the query string specifies the stock ticker whose current price is sought.Once the program determines the associated price, it will return the results to the Web server in the form of an HTML page. In turn, the server will return thepage to the user’s browser in exactly the same fashion that it would return anyother page.

In most real-world applications, HTML forms are used instead of URLs withquery strings. The details of how this is done are well beyond the scope of thisappendix. Readers who are interested in the details of CGI programming shouldsee Hamilton (2000).

WebServer

WebBrowser

HTTP

DynamicHTMLPage External

Data

Static HTML Pages

CGIProgram

EXHIBIT C.6 How CGI Works

query stringThe information thatspecifies various para-meters and values that aCGI program will useduring execution.

C-10 Appendix C

<HTML><HEAD><TITLE>E-Commerce</TITLE></HEAD>

<BODY BGCOLOR � “#FFFFFF”><H2>Retrieve Current Stock Price</H2><A HREF � www.stocknews.com/cgi-bin/stock.exe?ticker�msft>Microsoft</A><BR><A HREF � www.stocknews.com/cgi-bin/stock.exe?ticker�orcl>Oracle</A>

</BODY></HTML>

EXHIBIT C.8 HTML Code for Accessing Stock Price

EXHIBIT C.7 Accessing Stock Prices Through URLs

Appendix C C-11

ALTERNATIVE APIsAlthough CGI programs were widely used, they suffered from some significanttechnical limitations, especially at Web sites with a lot of traffic. CGI programs areeither separate executable programs or interpreted scripts. When a Web serverinvokes a CGI program, an individual copy of the program is brought into mem-ory. After the program has run, the executable program is unloaded from memory.If the server receives another request while the first program is still running, thenit will invoke a second instance. The number of copies in memory will depend onthe number of requests the server is trying to service. At a large site, the number ofcopies could run in the thousands. Obviously, the frequent loading and unloadingof a large number of copies is very inefficient, not to mention the fact that the per-formance of the server will degrade substantially as the number of requestsincreases.

Other gateway architectures and APIs have been developed to overcome thelimitations of CGI programming. Two alternatives are FastCGI and Microsoft’sISAPI (Internet Server API). The difference between these APIs and CGI is thatthe server invokes only a single instance of the program, and a technique known asmultithreading is used to handle individual requests (each request is now a threadinstead of the entire program). The overall result is increased efficiency and sub-stantially improved performance.

SERVER-SIDE SCRIPTINGWith the exception of the Perl programming language, it takes a skilled program-mer to write a Web-based application using one of the Web gateway APIs. Fortu-nately, simpler scripting languages can be used to accomplish many of the sametasks. These scripts are run on the server side, not the client side. Included amongthese specialized server-side scripting languages are Microsoft’s Active ServerPages (ASP), Apache’s Hypertext Preprocessor (PHP), Java Server Pages ( JSP),and ColdFusion’s Markup Language (CFML). All of these languages are usedwidely throughout the Web. For example, in April 2002, the Netcraft monthlyWeb server survey (netcraft.com/survey) estimated that approximately 48 percentof the 37 million sites it surveyed were using either ASP or PHP scripting (Berkes2002). Buser (2000) provides a good introduction to ASP scripting, and White-head and Desamero (2001) offers a good introduction to PHP scripting.

All of these server-side scripting languages work in the same manner. Basically,a server-side script is nothing more than a Web page that contains script statementsinterspersed with various HTML tags. The script statements are denoted by spe-cialized tags. Exhibit C.9 displays a very simple ASP page. The server-side scriptstatements are denoted by the <% . . . %> tags. The first script statement(“request.form”) captures the username entered on a form. The second script state-ment (“response.write”) simply writes the user’s name on the Web page. The rest ofthe statements in this script are nothing more than standard HTML tags.

When this page is accessed, the Web server first hands it off to the appropri-ate server-side script processor. How does it know which processor to use? Theappropriate processor is selected by the file extension of the page being accessed.For instance, an ASP page has an “.asp” file extension, whereas a PHP page has a

C-12 Appendix C

“.php” extension. The selected processor then executes the script code within thepage and sends the resulting page back to the browser via the Web server. Whenthe browser receives the page, it renders the rest of the HTML code. In this waythe content of the page is dynamically generated in much the same way that thespecial API programs generate dynamic content. The resulting page is shown inExhibit C.10 (assuming that the user entered the name John Smith).

EXHIBIT C.10 ASP Output

<HEAD><TITLE>E-Commerce</TITLE></HEAD><BODY><% username � Request.Form(“username”) %><% Response.Write username %><P>Thank you for joining us. Would you like to seeyour orders?<FORM METHOD � POST ACTION�”orders.asp”><INPUT TYPE � “submit” VALUE � “Yes”></FORM></BODY>

EXHIBIT C.9 ASP Code

Appendix C C-13

REFERENCESBerkes, D. “Survey Says: PHP Passes Microsoft Active Server

Pages.” newsforge.com/newsforge/02/06/11/011243.shtml?tid�5 ( June 11, 2002).

Buser, D., et al. Beginning Active Server Pages 3.0. Birmingham,UK: Wrox Press, 2000.

Buyens, J. Web Database Development: Step By Step. Redmond,WA: Microsoft Press, 2000.

Hamilton, J. “CGI Programming 101.” cgi101.com (2000).

Harris, A. JavaScript for the Absolute Beginner. Indianapolis, IN:Premier Press, 2001.

Negrino, T., and D. Smith. JavaScript for the World Wide Web,4th Ed. Berkley, CA: Peachpit Press, 2001.

Whitehead, P., and J. Desamero. PHP: Your Visual Blueprint forCreating Open Source, Server-Side Content. New York:Hungry Minds, Inc., 2001.

KEY TERMSapplication programming

interface (API), p. C-1common gateway

interface (CGI), p. C-1

document object model (DOM), p. C-6

JavaScript, p. C-5

query string, p. C-10virtual machine

(VM), p. C-2

ACCESSING RELATIONAL DATABASESRelational databases are an integral part of virtually every EC site. Most of thepages that appear on the Web are dynamically generated from database content.This is one of the major roles of server-side scripting. Scripts are written to processthe input from an HTML form, to use the input to construct a relational databasequery behind the scenes (i.e., to produce an SQL statement that the relationaldatabase can execute), and then to dynamically produce and return an HTMLpage from the results of the query.

For example, you may have used the FedEx or UPS Web sites to track a ship-ment. The status of any shipment for a given customer is contained in a database.To determine the status of a package, the customer enters the shipment number ina Web form. When the customer clicks the submit button, the shipment numberis passed to a server-side script. The script takes the shipment number and con-structs a relational query (e.g., SELECT shipment_no, shipment_status FROMshipments) that is passed to the shipment database. The database produces a resultcontaining the shipment_status, which is passed back to the script. The scripttakes the results and generates an HTML page on the fly, which is then returnedto the customer.

Again, the specific techniques that are used to dynamically access the informa-tion in a relational database are beyond this discussion. Readers who are interestedin this topic should refer to Buyens (2000).