To use an alternative database other than the built-in HSQL DB to back your entity beans, you need to first define the data source for the database and register it in the JNDI. This is done via the *-ds.xml files in the deploy directory. Examples of *-ds.xml files for various databases are available in
<JBOSS_HOME>/docs/examples/jca directory in the server.
Then, in the persistence.xml, you need to change the jta-data-source attribute to point to the new data source in JNDI (e.g., java:/MysqlDS if you are using the default mysql-ds.xml to setup a MySQL external database).
In most cases, Hibernate tries to automatically detect the database it connects to and then automatically selects an appropriate SQL dialect for the database. However, we have found that this detection does not always work, especially for less used database servers. We recommend you to set the hibernate.dialect property explicitly in persistence.xml. Here are the Hibernate dialect for database servers officially supported on the JBoss platform.
Oracle 9i and 10g: org.hibernate.dialect.Oracle9Dialect
Microsoft SQL Server 2005: org.hibernate.dialect.SQLServerDialect
PostgresSQL 8.1: org.hibernate.dialect.PostgreSQLDialect
MySQL 5.0: org.hibernate.dialect.MySQL5Dialect
DB2 8.0: org.hibernate.dialect.DB2Dialect
Sybase ASE 12.5: org.hibernate.dialect.SybaseDialect