Flat File Database

A flat file database is a database which is stored on its host computer system as an ordinary, non-indexed flat file. To access the structure of the data and manipulate it, the file must be read in its entirety into the memory of the computer. Upon completion of the database operations, the file is again written out in its entirety to the file system of the host. In this stored mode, the database has no structure which can be seen external to the file and thus is referred to as a flat file.
The term has generally implied a small, simple database. As computer memory has become cheaper, larger and more sophisticated databases can now be held in memory in their entirety for faster access, but these would not generally be referred to as flat-file databases.

You may hear about xBase, dBase, Btrieve, or other databases as you work with your applications. These databases are prime examples of the older technologies that traditionally employ flat file approaches. Since the "early days," a whole 17 or so years ago, many of these databases have employed engines that let you query their database with SQL.
XBase uses an approach of creating a file for the data, a set of header information that describes how the data is formatted in the database, and one or more separate index files, typically updated by the system.
It is not unusual to have an xBase system that has tens or hundreds of database files. With SQL, these files map to tables, which you will learn more about in the coming lessons. With a relational database system, these xBase databases are rolled together inside the overall application database.