What's the alternative of MySQL ? Because MySQL may be closed-source

The decision is certainly Sun Microsystem's business. However, the users have an opportunity to help Sun Microsystem's understand the consequences of their decision.

For instance, if you want to continue using an open-source database, PostgreSQL might be an alternative worth looking at. Developing for the Microsoft platform, it offers native interfaces for C, C++, and .NET, and even the "good old" (?) ODBC is available.