The word schema as defined in the dictionary means plan, diagram, scheme or an underlying organizational structure. Therefore, as can be very briefly said, an external schema is a plan on how to structure data so it can seamlessly integrates with any information system that needs it.
It also means that data needs to integrate with the business schema of the implementing organization. External Schema is a schema that represents the structure of data used by applications.
A database management system typically has a three layer architecture composed of internal schema, conceptual schema and the external schema. The conceptual schema is the schema which describes the aspects relevant the universe of discourse in the DBMS.
Each of the external schemas describes the part of the information which is appropriate to the group of users at whom the schema is being addressed. Each external schema is derived from the conceptual schema.
The external schema definitions are all based on a data dictionary. The universe of discourse of the data dictionary is all information in the use and management of the database system.
The external schema definition system is the means by which the external schemas are being defined. The external schema must contain information which must be derivable from the conceptual schema.
In systems which are based on the object oriented paradigm, this may not necessarily mean that the classes which have been included in the schema have to have been previously defined in the conceptual schema. Any external schema may also include classes that have been defined in the conceptual schema like it may also contain derived classes.
A derived class could be any classes which have been directly or indirectly defined on the bases of the conceptual schema classes and have been defined and included in the data dictionary.
The typical external schema definition for the object oriented paradigm includes three steps which are: definition of the necessary derived classes; selection of the set of classes that will constitute the external schema; and generation of the output external schema.
A general external schema explicitly defines data structure and content in terms of the data model that tackles structural, integrity and manipulation of the data. As such, the external data schema include the data vocabulary which defines the element and attribute names, the content model which holds the definition of relationships and corresponding structure, and the data types.
Some of these data types are integer, string, decimal, boolean, double, floast, hexBinary, base64Binary, QName,dateTime, time, date, gYearMonth, gYear, gMonthDay, gDay, gMonth, NOTATION and all others which may be applicable or appropriate for representing the entities in the information system.
An external schema is a very important aspect of any information technology application most especially in the field of database.
As this defines the structure of data used in applications, this will make it easy for the information system to deal with all processes, optimizations and troubleshooting. This schema will also ensure that the data used in the information system adheres by the rules of the business and follows the framework of the data architecture.
In data modeling, the external schema may refer to the collection of data structures that are being used in creating databases representing all objects and entities which are modeled by the database.
The same external schema stores the definition of the collection of governing rules and constraints place on the data structure in order to maintain structure integrity by preventing orphaned records and disconnected data sets. In the same external schema is also can be found the collection of operators which are applicable to the data structures such as update, insert, query of the database.