A database is a collection of data, which is categorized in some way. The data has to be quickly accessible and modifiable for optimal categorization. Relational databases are the most commonly used databases. Data in them is placed in tables and they are accessed usually with SQL (Structured Query Language). The are many different types of databases from proprietary one like Oracle to free ones like MySQL. Databases are the heart of dynamic Web pages. All dynamic Web pages must get their information from a database.

Below is a partial list of some of the more popular databases. Please Note, in addition to the listings below, many of the commercial database vendors have free "personal edition" or "single user" policies that allow their products to be used free of charge at home or in non-commercial applications. The goal of these policies is to allow users to evaluate their software free-of-charge, and to allow developers to port their applications without making a large financial investment.

PostgreSQL
PostgreSQL is a powerful and complete SQL implementation that rivals commercial databases such as Oracle. This database (along with MySQL) is the leading GPL'ed database system for the GNU/Linux & BSD platforms. Commercial support is available from a company formed by the leading Postgres developers.

This database is fully Transactional, with full ACID (Atomicity, Consistency, Isolation and Durability) properties. There are seven different levels of locking for consistent queries and updates. Tremendous performance improvements in Version 7.0 Support for Triggers Support for user defined data types. Support for stored procedures. These can be defined in a variety of ways, including an internal language, or very generally as a shared library, written in any language, linked into the Postgres server. Includes Online Backups, Multiple index type (b-tree, hash, etc.), Commercial ODBC drivers from OpenLink, Non-commercial ODBC drivers. The Postgres HOWTO provides an extensive reference for a large number of Postgres facts and tools.

mySQL
MySQL from MySQL AB is, with Postgres, one of the most popular GPL'ed databases for the GNU/Linux and BSD platforms.

Commercial sales, support and training is available from NuSphere. They also do contract programming and infrastructure enhancements and feature additions for MySQL. Includes commercial support from AbriaSoft. Rudimentary support for table locking recently added. Also has complex but powerful user access control mechanism. Multi-threaded for parallel queries Includes ODBC, Perl, C, Java, Tcl, Python API's

InterBase
The InterBase FireBird SQL server from InterBase Corp. is the well-known InterBase database released under an open source (MPL) license. In case you are wondering why you haven't heard of InterBase before, note that they were recently spun off from Inprise, which is, of course, the new name for Borland.

Commercial SQL Databases
There are a large number of database vendors that support Linux. It is difficult to review these accurately in a small space, yet, based on some highly subjective criteria, I've attempted just that. As you shop for a database, you should make your own list of the features and functions that you believe are important for your application.

Remember that the "best" database is not necessarily the one with the greatest number of features, or the lowest price. For your needs, it may be raw performance for small queries, or it may be ease of administration of a database with tens of millions of records. Remember that whichever database you choose, you will be living with it for many years.

This section (of commercial databases) is no longer actively maintained. After all of the 'big boys' announced availability on Linux in 1998, so did everyone else, and it has become impossible/impractical to attempt to list all of the commercial databases available for Linux.