You can use the MySQL software for free under the GPL if
you adhere to the conditions of the GPL.
For additional details, including answers to common questions about the GPL,
see the generic FAQ from the Free Software Foundation at
http://www.gnu.org/licenses/gpl-faq.html.
Common uses of the GPL include:
MySQL
source code under the GPL with your product.
MySQL source code bundled with other
programs that are not linked to or dependent on the MySQL system
for their functionality even if you sell the distribution commercially.
This is called mere aggregation in the GPL license.
MySQL
system, you can use it for free.
MySQL servers for your customers.
We encourage people to use ISPs that have MySQL support,
as this will give them the confidence that their ISP will, in fact,
have the resources to solve any problems they may experience with
the MySQL installation. Even if an ISP does not have
a commercial license for MySQL Server, their customers
should at least be given read access to the source of the MySQL
installation so that the customers can verify that it is correctly patched.
MySQL database software in conjunction with a
web server, you do not need a commercial license (so long as it is not
a product you distribute). This is true even if you run a commercial
web server that uses MySQL Server, because you are not
distributing any part of the MySQL system. However, in this
case we would like you to purchase MySQL support because the
MySQL software is helping your enterprise.
If your use of MySQL database software does not require a commercial
license, we encourage you to purchase support from MySQL AB anyway.
This way you contribute toward MySQL development and also gain
immediate advantages for yourself. See section 1.4.1 Support Offered by MySQL AB.
If you use the MySQL database software in a commercial context
such that you profit by its use, we ask that you further the development
of the MySQL software by purchasing some level of support. We feel
that if the MySQL database helps your business, it is reasonable to
ask that you help MySQL AB.
(Otherwise, if you ask us support questions, you are not only using
for free something into which we've put a lot a work, you're asking
us to provide free support, too.)