One caveat on all of the below is that I haven't looked much at mysql in several years so the information may have changed somewhat.
In the broadest sense Oracle and Sybase are very similar. They are both enterprise database systems that will likely cost you a lot. Both can be turned in lots of ways to give you good performance for your intended usage. Mysql is not as robust a database and has extremely good read performance and poor write performance.
In terms of writing queries, as long as you stick with basic sql queries, all of them are more-or-less sql standard. The biggest difference you will encounter is that there are no standard implementations for handling dates and datetimes. If you need to write stored procedures or other more complex forms of sql, there are many more differences between them.
One unique quirk to Sybase is it supports putting multiple sql statements in a single command/submit and retrieving the results for all of the queries. This can be useful but I have yet to find a place where I couldn't structure the query differently (or just submit each statement separately) to get it to work in another database.
If price is a concern, go with mysql. If you will be reading data far more than writing data, go with mysql. If you need a full enterprise database, go with Oracle or Sybase. I've used both Oracle and Sybase and can't really recommend one over the other - both can do pretty much the same things.
