There is no such thing as a "server language". I assume you mean you want to learn SQL, which is a language for querying relational databases. Both SQL Server and Oracle are relational database management systems, so yes either one of those will work if you want to mess around with databases. You can also use MySQL or PostgreSQL, which are open source systems.
These all differ by what advanced features they offer. Generally PostgreSQL has the most advanced features, while MySQL tends to be the most basic. They all use SQL, though there will be some minor differences in syntax and what you can do (usually just with joins). SQL is a standard, though, so they will all be similar enough that it doesn't matter which one you learn.