WordPress uses MYSQL, which is released under GPL. So I think Workpress MUST be under GPL.
But my friend said WordPress doesn’t include MySQL program when it released, so it can be released under other license, and don’t need to open source.
Which is correct?
You can find it on the WordPress website, under “License”. It is GPLv2
MySQL is not a part of WordPress, WP simply uses it. Their licenses are unrelated.
The license under which the WordPress software is released is the
GPLv2 (or later) from the Free Software Foundation. A copy of the
license is included with every copy of WordPress, but you can also
read the text of the license here.
Part of this license outlines requirements for derivative works, such
as plugins or themes. Derivatives of WordPress code inherit the GPL
license. Drupal, which has the same GPL license as WordPress, has an
excellent page on licensing as it applies to themes and modules (their
word for plugins).
There is some legal grey area regarding what is considered a
derivative work, but we feel strongly that plugins and themes are
derivative work and thus inherit the GPL license. If you disagree, you
might want to consider a non-GPL platform such as Serendipity (BSD
license) or Habari (Apache license) instead.
WordPress was originally fork of b2/cafelog and inherited GPL license from it. See history in Codex.
It is explicitly under terms of GPLv2 or later license.
MySQL licensing doesn’t influence this in any case, because it is not a part of WordPress as software product (just like PHP, web server or operating system).