databases..

SQL is a language not a specific database. Do you mean MySQL? If so, people have certainly used it for commercial games.

-me

I have been playing around with databases and games for a while, this includes GUI Based and Console based (MUD). What I have found is that with most database engines the systems are too slow to access it directly. What I have done in the past is to store the information in a database for long time storage, but have a different worker process that syncronised the data stored in memory with the database backend.

Using a database for real time storage (i.e. storing regular variables or quickly changing game data) is not recommended. A database should be treated like a more advanced data file - something you can easily store and retrieve many discrete pieces of data from with a wide range of criteria. If you are writing a game server, a database is an ideal place to save persistent information. On the client side you could use a database as a more advanced pak file, though most databases are of a server-client design, meaning users would have to install and be running a seperate database program. A good alternative is SQLite, an extremely compact embedded database, which allows you to have a single database file that it manipulated by your program (if SQLite is compiled in) or by the SQLite DLL.

It really depends on what it's used for. What are you planning on putting into your DB?