An
application server lets you work with server-side resources such as databases.
For example, a dynamic page may instruct the application server to extract
data from a database and insert it into the page’s HTML.
The instruction to extract data from a database is called
a database query. A query consists of search criteria expressed in a database
language called SQL (Structured Query Language). The SQL query is written
into the page’s server-side scripts or tags.
An application server cannot communicate directly with a database because
the database’s proprietary format renders the data undecipherable in
much the same way that a Word document opened in Notepad is undecipherable.
The application server can communicate only through the intermediary of a
database driver. A database driver is software that acts like an interpreter
between the application server and the database.
After the driver establishes communication, the query is executed against
the database and a recordset is created. A recordset is a subset of data extracted
from one or more tables in a database. The recordset is returned to the application
server and the data used in the dynamic page.
Here’s a simple database query written in SQL:
SELECT lastname, firstname, fitpoints
FROM employees
This statement creates a three-column recordset and fills it with rows containing
the last name, first name, and fitness points of all employees in the database.
One can use almost any database with your web application,
as long as you have the appropriate database driver for it.
If you plan to build small low-cost applications, you can use a file-based
database, such as one created in Microsoft Access. If you plan to build robust,
business-critical applications, you can use a server-based database, such
as one created in Microsoft SQL Server, Oracle 9i, or MySQL.
If your database is located on a system other than your web server, make sure
you have a fast connection between the two systems so that your web application
can operate quickly and efficiently.
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