I have read the GPL, but still have something I don't understand.

I take Mysql just as an example (no intention to attack their company) :

1. As the software is distributed under GPL, do the company Mysql AB still own the copyright of the code ? If so, how about other contributors of the software ?

2. If it is GPL, why the company has the exceptional power to sell the software under other commercial license ?

3. Finally, does it mean that the original programmer has the right to alter the license as he / she think fit ?

1. The GPL gives you no less right to copyright a work. The code is copyrighted and released under the GPL.

2. You can re-sell any piece of GPL code at any price you like. You are obliged to provide, on request, the full source code to anyone who requests it, however.

3. No, this is why Microsoft describe the GPL as a virus. Once a piece of software is released under the GPL, it cannot be re-licenced with a less free alternative.

Thanks Martin.

I still have something don't understand.

If I found a GPL software is useful for me, I modified it for my personl / company use. Do I have any obligation to share my modification with the public if somebody ask for it even I did not distribute it ?

Hello!

zero reply since June 9th, 2002 till now ^^ Amazing
In fact, there are here a big mistake :
1 - Under any license, you still have copyright right, and it's because this one that you can put your creation under this license...

2- Licensee can't use the software if they want to do something prohibed by the GPL, but licensor can do whatever he want (double licenses are ok) with his original software.

3- the original programmer has the right to alter the license as he / she think fit , if he is the only creator. In the other cas, he is licensee of any contribution and have to respect the license for these contributions. I don't know how do the Mysql company, but it would not use other contribution in its commercial version!

P.S. There are a special GPL exception (FLOSS) wich permit any other free/open source software to use this library without becoming GPL themeself

Mben: yes, amazing noone corrected this before!

Yes, they can alter the license, as Mben says, but of course, they cannot "take back" any existing code released under the GPL or another FLOSS license. An example of this is X. The XFree86 team changed their license, which some people didn't like, so they split the code from the last version released under the old license and renamed it Xorg.

No, you don't need to release your personal modifications unless you release/distribute the software.