A RelationalDataBase extended to make it useful in the real world.

RelationalDataBases proved very powerful but somewhat hard to use for mortals, leading to their extension. Typical extensions include:

1. Procedural extensions to SQL. These lead to the current plathora of SQL variants--everyone extended SQL differently.
2. Extensions to the relation structure to allow for variable sized fields, particularly for string data. These have much less impact than the procedural extensions to SQL and are largely implemented by making fields pointers to the data rather than the data itself.

Most real-world databases are PostRelationalDataBases: Oracle, MySQL, PostgreSQL, Microsoft SQL Server, Informix, Borland Interbase, FireBird and IBM DB2.

See also: http://howfuckedismydatabase.com/