Everyone knows that you should reuse your code so that you don't have to repeatedly write the same functionality. You put widgets in custom tags and encapsulate logic in CFCs. Even so, your application's user interface may frequently end up changing. Sometimes the changes are so substantial that it hardly seems worth the effort to try to reuse code at all. Your efforts to reuse code and separate business logic from presentation aren't worthwhile unless you can write your presentation code so that it will be easy to change to fit changes to your clients' needs or other business requirement changes.

In this article, I will try to show a few techniques that I have found helpful in writing reusable code for application user interfaces. I'll use an example from a fictional search-reporting tool that was created in Dreamweaver and modified in HomeSite+.

Separate Logic and Presentation Code
If you look at Listing 1, the first thing that you might notice is that within the output loop I am querying the database for more information about each search term. This presents a number of problems, including the potential for very poor performance. The more pertinent problem, however, is that it clutters up the presentation code; making changes to this code is more confusing and tiresome (especially in cases where there is more logic code inside the output loop than is shown here).

The query logic (executed on each loop pass) can be pulled into the original query (see the SQL sidebar for more details). Pulling the logic into the original query improves performance (due to fewer connections to the database) and improves readability and maintainability of the code.

SQL
I would highly recommend that you learn and practice using some of the basic SQL aggregate functions such as GROUP BY and OUTER JOIN if you aren't already comfortable with them. The GROUP BY keyword is used in Listing 2 because it will return the results of aggregate functions across multiple rows by the columns in the GROUP BY statement.

All of this is standard SQL and therefore should work on most databases. Ben Forta's Teach Yourself SQL in 10 Minutes (Sams Publishing) covers this well, as does SQL for Dummies (Wiley Publishing). You may not want to learn from a Dummies book, but I first learned SQL from this book (after being introduced to it in Ben Forta's ColdFusion Web Application Construction Kit) and I think it is pretty good. Either book is a good choice - just remember that while the aforementioned CFWACK includes a good introduction to SQL, your life will be made easier by learning more.

Table 1 shows some standard aggregate functions. Since you are moving logic out of the output loop, look at this line:

<cfset PercentTotal = (numSearches / sumSearches) * 100>

This line also represents logic performed in the output loop. It is only one line so it probably isn't a major problem to leave it in the output loop. Even so, your life will be made easier in the long run if all of your logic is performed outside of the output. In this case, you can perform this calculation directly within the query itself - yes that's right, let the database do the work for you. (What if you couldn't? - see the Query Functions sidebar.)

As you can see from looking at Listing 2 (see sys-con.com/coldfusion/sourcec.cfm), the changes made the code much easier to read and, therefore, easier to maintain and modify.

Query Functions
In this ca