Linux users have their choice of good relational databases. However, some applications require an approach that takes advantage of object-oriented programming. Here's some advice on when and where to use a native object database like db4o instead of a relational database.
If you want an open source database that runs on Linux, you have plenty of options to choose from. MySQL and PostgreSQL are good choices for medium- to large-scale applications, while compact alternatives such as HSQLDB work for small and embedded systems. All these are relational databases, and they are good at what they do -- handling data. This is fine if you take a data-centric view, where your application's job is simply to perform a series of database transactions.
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