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

. |02.04 - Conceptual Model                          .

  Conceptual  model  abstraction  pattern  basis  for
Relation  Database  Structure   manipulation  is  the
original idea of elements  organized by it's purpose.
The  main  author  in  this  field  is  E.  F.  Codd.
When  he  published  his article  on  relation  model
implementation  using mathematical  apparatus he  was
employed in the IBM research laboratory.

  He was looking for method how to separate user from
the  data  organization likewise  deductive  programs
already  allowed  that.  He  defined  requirement  to
keep  the  work  sequence   even  when  the  database
inner  structure is  changed.  Like  this he  enabled
to  separate the  user's work  sequence for  database
structure manipulation from the inner organization of
data base.

  Codd's  presumption was  the dynamical  variability
of  inner organization  as  result  of frequent  data
updates,  output  reports modifications  and  natural
growth  of the  information stored  in the  database.
By  separation of  the  user's  data processing  from
the  inner  machine  data  representation  and  inner
mechanisms Codd achieved the preservation of the work
sequences with the data base.

  The  separation of  information structure  from the
user's interface was implemented in the form of query
language.  Query language  didn't  have consider  the
order of each  elements in the data  structure on the
physical level of  database. He introduced principles
allowing faster  access to the bigger  amount of data
and he  specified the  method of abstraction  for the
data access.

  He  did  also engage  in  the  future direction  of
query  language development.  As the  base functional
apparatus for  the query  language he  used predicate
calculus.  That brought  him to  the idea  to connect
argument with  the stored  language even in  the case
when  relation is  not  know but  exists. He  defined
as  the base  query  language  property it's  ability
to  be  encapsulated  in  different  host  languages.
The  basic  principle   of  data  language  principle
became  it's description  ability. That  brought it's
characteristics in  the term  of ability not  only to
formulate data  structure, but  by the  same language
constructs manipulate with the data structure.

  Data  Model is  connected  to  the data  structure,
operations  and operations  constraint.  One of  it's
classification is based on  the level of abstraction.
Two  main  levels  of abstractions  are  logical  and
conceptual.  On the  logical  level  it's the  user's
logical  structure  knowledge,  primary  and  foreign
keys existence.  There are  constraints for  the data
reorganizations and data structure.

  On   the  logical   layer   there   is  an   Entity
Relationship model and logical  data structure as are
keys, foreign  keys, constraints and  entities joins.
On  the conceptual  level  there is  ER model  that's
widely used for conceptual modeling.

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