Database Classifications
There are several ways of classifying the database systems available for Linux:
- Based on "freeness." - There are commercial packages (sold for a price), and there are free software database systems (that do not cost anything to acquire). This is fairly closely correlated to availability of usable source code, which is another legitimate interpretation of "freeness."
- Compatibility/Means of Functioning -
- xBASE - This is traditionally a "PC" oriented system, of which the first version was known as Vulcan, and ran under CP/M . It was later renamed dBase, and later versions and competitors have followed.
The system model generally involves data structured in a dual fashion similar to ISAM databases with "data" files containing data, and "index" files containing index information. Applications access data directly by reading the files. Newer versions have a network locking system to manage contention for files and/or records if multiple users try to access data simultaneously, but there is still contention inherent in that many programs are accessing the same files simultaneously.
More modern systems use "extent-based" allocation systems to better support the handling of tuples of varying sizes. - SQL - Structured Query Language - Ingres was the progenitor of the modern "query language," with its QUEL query language; a similar query language was then designed that we now know as SQL. SQL is arguably inferior to QUEL, as QUEL had a syntax that is simultaneously simpler and more powerful than that of SQL.
The system model typically involves there being a central database manager "engine" or "process;" application programs do not have direct access to the data. This allows data to be relatively protected from corruption/misuse by rogue processes.