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  _________________  .____     ___________________
 /   _____/\_____  \ |    |    \_   _____/\______ \
 \_____  \  /  / \  \|    |     |    __)_  |    |  \
 /        \/   \_/.  \    |___  |        \ |    `   \
/_______  /\_____\ \_/_______ \/_______  //_______  /
        \/        \__>       \/        \/         \/


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. |02.01 - DBMS and Database Layers                  .
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  Previously it was shown  where the database systems
came from. In the beginning there was need to process
natural language  questions and get  natural language
answers from computers.

  On  the basis  of Deduction  Systems the  Data-base
systems  were   built  by  separating   data  storage
implementation from data processing algorithms.

  We  will  now try  to  show  some basic  terms  and
principles needed for databases understanding.

  In the core of the  standard database is a Database
Management  System (DBMS).  It's  designed to  manage
data  sentences  and  it's   capable  to  handle  the
database  growth  and  it's  structure  change.  DBMS
enables  data structure  specification.  It takes  in
account the  type of stored information.  It provides
mechanism for data manipulation.

  If  there's need  to share  information among  many
users  it's  task is  to  avoid  anomalies caused  by
concurrent  access by  many users  to the  same data.
Database System enables abstract  view on the data by
giving abstract layer above the data storage and data
management details.  Data in data base  can be stored
in  complex data  structures. But  yet DBMS  provides
effective interface for stored data access.

  Typical database model provides 3 main views on the
stored data. It can be  sorted from the lowest to the
highest  from  the  abstraction point  of  view.  The
levels can be called physical, logical and user's. On
the  lowest  level  of abstraction,  physical  level,
there is description  on how the data  are stored and
there  are  details on  the  data  structure. In  the
middle layer, logical level,  there is described what
data are stored and what's  the relation among it. In
the highest level of  abstraction, user's level view,
the users  access is allowed and  provides user's own
view to  the data.  Application program is  a program
using access to the  database and sometimes is placed
in it's own application layer.


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