Personally I would learn Oracle SQL. The largest companies all run Oracle, and its the easiest and most marketable skill in my opinion. SQL is a standard, although Oracle has its own keywords and practices. If you learn Oracle SQL, you can adapt that to other databases with ease.
I would also recommend learning PL/SQL as most development requires some extra programming. Personally speaking I would stay away from the Microsoft route and go with Oracle! Every company needs reports run, so you can start at the bottom programming reports and move up the ladder.
As far as platforms, dont worry so much about learning Unix if you dont know it. Stick with learning the SQL part of it, and you can learn the Unix as you go. Ive done fine and know little to no Unix.