eXtropia: the open web technology company
Technology | Support | Tutorials | Development | About Us | Users | Contact Us
 ::   Tutorials
 ::   Presentations
Perl & CGI tutorials
 ::   Intro to Perl/CGI and HTML Forms
 ::   Intro to Windows Perl
 ::   Intro to Perl 5
 ::   Intro to Perl
 ::   Intro to Perl Taint mode
 ::   Sherlock Holmes and the Case of the Broken CGI Script
 ::   Writing COM Components in Perl

Java tutorials
 ::   Intro to Java
 ::   Cross Browser Java

Misc technical tutorials
 ::   Intro to The Web Application Development Environment
 ::   Introduction to XML
 ::   Intro to Web Design
 ::   Intro to Web Security
 ::   Databases for Web Developers
 ::   UNIX for Web Developers
 ::   Intro to Adobe Photoshop
 ::   Web Programming 101
 ::   Introduction to Microsoft DNA

Misc non-technical tutorials
 ::   Misc Technopreneurship Docs
 ::   What is a Webmaster?
 ::   What is the open source business model?
 ::   Technical writing
 ::   Small and mid-sized businesses on the Web

Offsite tutorials
 ::   ISAPI Perl Primer
 ::   Serving up web server basics
 ::   Introduction to Java (Parts 1 and 2) in Slovak


Introduction to Databases for Web Developers
As we have already discussed, relational databases are table based. That is, all SQL commands are applied to the contents of tables and then those results are displayed as tables themselves. One great consequence of that is that it is fairly easy to perform queries upon the virtual tables created by other queries. Or, in jargonese, to perform subqueries.

Consider this example in which we want to get a listing of all the clients who made a transaction of greater than $150.00.

	WHERE S_AMOUNT > 150.00

We would expect the following results:

    Jason Lim		456-7890
    Stephen Petersen	167-3333

As you can see, the subquery in the WHERE clause would return a set containing "101" and "103". Next, the top-level query would return the two clients. In this case, the DISTINCT operator was not necessary since we don't have many sales in our SALES table. However, for most subqueries, it is useful to include the distinct so that you do not get a client repeated for multiple sales. of course, you might want that to :)

Previous | Next | Table of Contents