| For a RDBMS there are two levels of abstractions of how data being stored and represented in the system: |
| 1) Logical database consists of the conceptual view of a DB system which is described by an ER (entity-relationship) model, defined and access by SQL and represented by tables,columns,views and other abstractions of data-object; |
| 2) The physical database consists of the actual phyical view of the DB which is represented by files,blocks,indexes,clusters,partitions,etc. |
| End-user and developers only need to deal with the logical level, |
| While the DBMS and a DBA (database administrator) define and perform the mapping between the two levels. |
| This is the reason why SQL achieves portability (oompared to f77), from a viewpoint of a data independent model. |