Date's Tweleve Rules for Distributed Database Systems - Database [DBMS] Independence
The system must support any vendor's database product. The DBMS instances at different sites all support the same interface - they do not necessarily all have to be copies of the same DBMS software. The ideal distributed system should be heterogeneous and should provide DBMS independence.
For example, if Ingres and Oracle both supported the official SQL standard, the Ingres site and the Oracle site might be able to talk to each other in a distributed database system. Pictorial representation of the same is given below.
|Fig 1 - Distributed database with database independence (source: Prof. Sin Min Lee ppt from CJ Date book)|