The three-schema approach , or three-schema concept , in software engineering is an approach to building information systems and systems information managementthat originated in the 1970s. It proposes three different views in systems development, with conceptual modeling being considered the key to achieving data integration . 
The three-schema approach provides for three types of schemes with schema techniques based on formal language descriptions: 
- External schema for user views
- Conceptual schema integrates external schemata
- Internal schema that defines physical storage structures
At the center, the conceptual schema defines the ontology of the concepts as the users think of them and talk about them. The physical schema according to Sowa (2004) “describes the internal forms of the data stored in the database , and the external schema defines the data of the application programs .”  The framework allows for the use of external schemata. 
Over the years, the skill and interest in building information systems has grown tremendously. HOWEVER, For the most part, the traditional approach to building systems HAS only Focused on defining data from two separate views, the “user view” and the “computer view”. From the user’s point of view, which will be referred to as the “external schema,” the definition of data in the context of reports and screens designed to help individuals in their specific jobs. The required structure of data from a user view changes with the business environment and the individual preferences of the user. From the computer view, which will be referred to as “internal schema”, data is defined in terms of file structures for storage and retrieval. The required data processing for data storage . 
These two approaches are based on the use of the data in the context of the application. In the creation of redundant and often inconsistent definition of the same data. Data was defined by the layout of physical records and processed sequentially in early information systems. The need for flexibility, however, led to the introduction of Database Management Systems (DBMSs), which allow for random access of logically connected pieces of data. The logical data structures within a DBMS are typically defined as either hierarchies, networks or relationships. Although DBMSs have greatly improved the shareability of data, the use of a DBMS alone does not guarantee a consistent definition of data. DBMSs and still have the problems of redundancy and inconsistency. 
The recognition of this problem led the ANSI / X3 / SPARC Study Group on Database Management Systems to conclude that in an ideal data management environment a third view of data is needed. This view, which is referred to as a “conceptual schema”, is a single integrated definition of the data within an enterprise. The objective of this conceptual scheme is to provide a definition of the integrity and integrity of data. 
The concept of a three-schema model consistant of a conceptual model , an external model, and year internal or physical model Was Introduced first by the ANSI / X3 / SPARC Standards and Requirements Planning Committee directed by Charles Bachman in 1975. The ANSI / X3 / SPARC Report feature DBMSs as having a two schema organization. That is, DBMSs utilize an internal schema, which represents the structure of the data as viewed by the DBMS, and an external schema, which represents various structures of the data as viewed by the end user. The concept of a third schema (conceptual) was introduced in the report. The conceptual schema represents the basic underlying structure of a company. 
The ANSI / SPARC report was intended as a basis for interoperable computer systems. All database vendors adopted the three-schema terminology, but they implemented it in incompatible ways. Over the next twenty years, various groups have attempted to define standards for the conceptual schema and its mappings to databases and programming languages. Unfortunately, none of the vendors had a strong incentive to make their formats compatible with their competitors’. A few reports were produced, but no standards. 
As the practice of data administration has evolved, the term “schema” has given way to the term “model”. The conceptual model of the relationship between end-users and database administrators covering those entities on which it is important to keep data, the meaning of the data, and the relationships of the data to each other. 
One further development is the IDEF1X information modeling methodology, which is based on the three-schema concept. Another is the Zachman Framework , proposed by John Zachman in 1987 and developed ever since in the field of Enterprise Architecture . In this framework, the three-schema model has evolved into a layer of six perspectives. In other Enterprise Architecture frameworks some kind of view model is incorporated.
- Conceptual schema
- Data model
- Data modeling
- Entity-relationship model
- Information systems
- Object-role modeling
- View model
This article incorporates public domain material from the National Institute of Standards and Technology website http://www.nist.gov .
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