Editing Database (section)

===External, conceptual, and internal views===

[[Image:Traditional View of Data SVG.svg|thumb|upright=1.15|Traditional view of data<ref name="ITL93">itl.nist.gov (1993) [http://www.itl.nist.gov/fipspubs/idef1x.doc ''Integration Definition for Information Modeling (IDEFIX)''] {{Webarchive|url=https://web.archive.org/web/20131203223034/http://www.itl.nist.gov/fipspubs/idef1x.doc |date=2013-12-03 }}. 21 December 1993.</ref>]]

A database management system provides three views of the database data:

* The '''external level''' defines how each group of end-users sees the organization of data in the database. A single database can have any number of views at the external level.
* The '''conceptual level''' (or ''logical level'') unifies the various external views into a compatible global view.{{sfn|Date|2003|pages=31–32}} It provides the synthesis of all the external views. It is out of the scope of the various database end-users, and is rather of interest to database application developers and database administrators.
* The '''internal level''' (or ''physical level'') is the internal organization of data inside a DBMS. It is concerned with cost, performance, scalability and other operational matters. It deals with storage layout of the data, using storage structures such as [[Index (database)|indexes]] to enhance performance. Occasionally it stores data of individual views ([[materialized view]]s), computed from generic data, if performance justification exists for such redundancy. It balances all the external views' performance requirements, possibly conflicting, in an attempt to optimize overall performance across all activities.

While there is typically only one conceptual and internal view of the data, there can be any number of different external views. This allows users to see database information in a more business-related way rather than from a technical, processing viewpoint. For example, a financial department of a company needs the payment details of all employees as part of the company's expenses, but does not need details about employees that are in the interest of the [[human resources]] department. Thus different departments need different ''views'' of the company's database.

The three-level database architecture relates to the concept of ''data independence'' which was one of the major initial driving forces of the relational model.{{sfn|Date|2003|pages=31–32}} The idea is that changes made at a certain level do not affect the view at a higher level. For example, changes in the internal level do not affect application programs written using conceptual level interfaces, which reduces the impact of making physical changes to improve performance.

The conceptual view provides a level of indirection between internal and external. On the one hand it provides a common view of the database, independent of different external view structures, and on the other hand it abstracts away details of how the data are stored or managed (internal level). In principle every level, and even every external view, can be presented by a different data model. In practice usually a given DBMS uses the same data model for both the external and the conceptual levels (e.g., relational model). The internal level, which is hidden inside the DBMS and depends on its implementation, requires a different level of detail and uses its own types of data structure types.