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.
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).