Stored data

Each different type of DBMS will have its own method of interaction with the OS. One DBMS cannot act with a database that is of a different type to the one it is designed for. A relational DBMS will probably have its own method for placing tables in files giving no choice in how data is stored, only the indexing of columns for searching.

To enable data transfer between databases it is necessary for DBMS to be able to use export and import (load and unload) functions. If the DBMSs are the same then the data is transferred to a file format that this particular DBMS can read. If there is a different DBMS then the transfer will be to a common file format that both DBMS can read.

If it is necessary to change an existing DBMS once again the data must be exported to a file format common to all DBMS.

Three schemas

The three schemas noted above are general, developed prior to relational databases. For DBMS using SQL there is nothing to be defined for a storage schema. Indexes are a possible exception. Grey area this. Very foggy! (A DBMS without a storage schema may appear to keep definitions simple, but as a consequence the choice of tables and their columns in a logical schema has to take account of requirements for efficient processing, which makes this task more difficult.) (What does that mean!!??)

A logical schema is concerned with tables (base tables) whose data are stored in a database. The properties of the base tables constitute a logical schema for a relational database. Any view of derived data is not part of the logical schema - it is not stored and must therefore be part of the external schema as it is defining data for a user process. This is mapped from the logical schema by an SQL statement. Not all tables available to user processes are views, most are base tables. SQL data definition allows the definition of views mapped from base tables in a logical schema.

Distinct components

Not all software packages separate out components. A DBMS may be executed as a single package with a database tool. User processes may be combined with a DBMS. However, for our purposes they should be considered as separate entities.

Databases can be centralized to a single computer or distributed as in a network. Hence the terms client application (user process) and SQL server (DBMS).

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Stuart Banner