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