Well, I'm not sure if you can write directly to the database (although that is intriguing, and I'll have to think about it a little more), but I can definitely say that it isn't necessary to use WinCC or an OPC Server.
If you are capable of writing your own middleware program, you could send PLC data via sockets or UDP, and let your app write to the SQL database (there are plenty of threads in this forum about how to do that). That is the simplest and quickest solution I can think of right at the moment.