MySQL isn't the best for learning SQL beyond basic queries, since it doesn't fully implement the SQL spec in the current release version.
Oracle would be nice, more of a pain to install and configure though. Not sure how easily it installs on a mac, but on solaris I had to make changes to /dev/proc and tweak the memory settings or Oracle would not even load.
As far as medium-duty databases go, MS SQL is actually one of my favorites... never had problem with an MS SQL server, and setting up data links between them is easy.
We've had Oracle crap out a TON of times at work, including some issues that even Oracle consultants couldn't figure out.
Also keep in mind that Oracle didn't even support the "INNER JOIN" syntax until version 9 I believe, and there may be other differences between Oracle and MS SQL today, which may cause headaches in your assignments.
Doing outer joins in oracle is a different syntax than MS too.