Instead of writing our own database, an existing database is used. There are many public available databases: Libdb, Mysql, and Postgresql are the most common. Unfortunately, each database has its own access library. To solve this issue, several people have written global database access libraries. When looking for a meta library, the things it should have are:
It should be easy to prgram.
It should not require many other libraries.
It should support as many databases as possible.
One of these meta libraries is iODBC (http://www.iodbc.org/). It also has a nice C++ wrapper called Sqlxx (http://www.ailis.de/~k/projects/sqlxx/). There is no particular reason in chosing excactly these libraries, excapt that they fulfill the requirements mentioned above.