Chapter 12. Accessing a Database

Almost all the web applications that you see on the Internet access a database. Databases store customer information, order information, product information, even discussion forum messages—in short, all information that needs to survive a server restart and is too complex to handle in plain-text files.

There are many types of databases used in the industry today. However, relational databases are by far the most common. A relational database uses tables to represent the information it handles. A table consists of rows of columns, with each column holding a single value of a predefined data type. Examples of these data types are text data, numeric data, dates, and binary data such as images and sound. A specialized language called Structured Query Language (SQL) is used to access the data. SQL is an ANSI standard and is supported by all major database vendors.

Relational database engines come in all shapes and sizes, from simple one-person databases with limited features, to sophisticated databases capable of handling large numbers of concurrent users with support for transactions distributed over multiple servers and extremely optimized search algorithms. Even though they all use SQL as the data access language, the API used to execute SQL statements is different for each database engine. To help programmers write code that's portable between database engines, the standard Java libraries include an API called the Java Database Connectivity (JDBC) API. JDBC defines a set of classes that can execute SQL statements the same way in any relational database.

The complexity of databases varies extensively. A database for an online discussion forum, for instance, requires only one or two tables, while a database for a human resources system may contain hundreds of related tables. In this chapter, we look at a set of JSTL database actions you can use to build any type of database-driven web application. But if the database is complex, you may want to use another approach: hiding the database behind application-specific beans and custom actions, or moving all database processing to a servlet and using JSP only to show the result. Both these approaches are discussed briefly at the end this chapter and in more detail in Chapter 18, Chapter 19, and Chapter 24.