Writing data in web database applications requires different techniques than reading data. Issues of transactions and concurrency become important, and we introduce these issues and the principles of dealing with them in this tutorial. The introduction is practical: we focus on the basic management techniques of locking and unlocking tables, and how to safely implement simple database writes in MySQL when there is more than one user simultaneously accessing a database. Most importantly, we identify when special approaches are required, and when these can be safely omitted from a web database application.
We begin by discussing a <form> designed to capture input for database writes. We also include more simple example scripts that illustrate more about PHP and its use in processing <form> input. We discuss some of the problems of <form> submission and validation further in the next tutorial.
We also include in this tutorial an example illustrating the reload problem, where variables and values are resubmitted when a web page in a browser is, for example, resized. This has practical problems-such as inadvertently buying two bottles of wine-and we discuss a solution that uses HTTP headers.
We then discuss how files can be uploaded from a web browser to a web server and the data then inserted into a MySQL table. We use as an example the uploading of GIF images of maps of wine regions. We also show how these images can be displayed using SQL queries.
By the conclusion of this tutorial, we will have covered the skills to build a simple but complete web database application. Several advanced topics remain, including validation of user-supplied data, adding state to a web database application, and authenticating users. We cover these three topics in the next three tutorials.