You can't create a database within a database. But you can create new tables within your database that have a different table prefix so that it will work like a different database.
All you have to do is make all your tables with a different table prefix.
I did this when I was running phpBB 2.0.23 and Simple Machines Forums on the same database. You just have to make sure that you call information from the right tables. It gets really confusing when you get more table prefixes in the same database.
This is a useful trick for free web hosting plans that only give you one database to work with.
in a database you can create tables.
into the tables you can create fields.
there is no sense to create a database
inside a database.
You want to build an online database manager program like google docs? That is emulating a database, and an emulated database is inside a database.
4 example: if you have an ACCESS database, you can build the same structure in mysql with a populator script. You see, i've thought further the question and a totally different solution would solve the problem.