Do I have to buy MySQL license? I've read in a forum that if your database capacity is more than 1G you should buy its license. Is this true? How much will it cost?
Could you post a link to the forum you're citing, I'd like to read the full context of the post.

I am not a lawyer, but legally speaking you don't need a license to use MySQL community edition. They do offer a paid-for enterprise edition. There are some additional tools you get, but I think the big thing people would use that for is support.

I've talked to some people that worked for companies that weren't allowed to use MySQL by veto from their legal dept, because they couldn't comprehend the concept of a license you don't pay for O_o.  

MySQL Community Edition flat out says it's free to use and licensed under the GPL.