MYSQL is free, but look at MYSQL’s weird licensing and it is not opensource.
And fact is that MYSQL it not free for all! I don’t understand their policy, but I know that using their dlls is prohibited to commericial customers only.
http://www.mysql.com/company/legal/licensing/
For Open Source Projects:
If you are developing and distributing open source applications under the GPL License, then you are free to use MySQL under the GPL License.
If you are developing and distributing open source applications under an OSI-Approved License, but not the GPL, MySQL provides the GPL License with a FLOSS Exception.
For Commercial OEMs, ISVs and VARs:
For OEMs, ISVs, and VARs who distribute MySQL with their products, and do not license and distribute their source code under the GPL, MySQL provides a flexible OEM Commercial License.
For Web Sites, Enterprise IT, and Government IT:
MySQL Network is the comprehensive subscription offering that provides database developers and DBAs with everything they need to successfully develop and deploy database solutions with MySQL. It includes Certified Software, updates and upgrades, proactive alerts and advisors, the online MySQL Knowledge Base, and full production-level technical support. The Certified Software (database server, connectors) is provided under the GPL License. Optionally, customers may choose a Commercial License.