First promo codes are only for new customers, so when you renew you'll do so a full price- which is still a good deal.
Im not sure what your question is about mysql. A website is not necessarly dependent on mysql, so if you don't need another database then don't create one. You're allowed to create as many DBs as you want, so if you need one then go ahead.
You have two options, you should be able to add on another database at your existing hostname, or you can create a new hostname and database. It makes no difference in the end.
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