MySQL

MySQL is a multithreaded database system created by MySQL AB, which is one of the most used database systems in the world. Now owned by Sun Microsystems its server and client-library are distributed under the GPL (which prohibits use by proprietary code) with a commercial licence to exempt from it.

MySQL has been a popular choice for PHP-based web-applications due to its ubiquity, but has been highly criticised for its many quirks.

Links

 * MySQL's homepage
 * MySQL on the Wikipedia
 * "Links against MySQL"