I am just starting to learn Ruby, and I want to do something that required a DB but I'm unsure whether MAMP would work or do I need something else?
Issues to consider:
Any libraries, gems, or other dependencies that expect Apache or MySQL to be running in a certain place might be thrown off by using MAMP.
On OS X, it's possible to run MAMP and the default version of Apache at the same time. This is easily fixable, but it might throw you off if you don't realize what's going on right away.
Any docs or tutorials are going to require to jump through an additional mental hoop, because your setup and requirements are going to be slightly different than whatever is the listed default for you system.
I haven't tried it yet, but I've been tempted because MAMP is a pretty good out of the box solution to get up and running quickly. But given some of the edge issues, I've also thought that just installing and running a more "normal" setup would be the better bet in the long run.