External view level:
The external view level is closest to the users. It is concerned with the way the data is viewed by individual users. You can say that external level is the individual user level. A user can either be an application programmer or an end-user, but DBA is an important special case.
The external level consists of many different external views of the database. Each external view describes the part of the database that a particular user group is interested in and hides the rest of the database from that user group. In addition, different views may have different representations of the same data. For example, one user may view date in the form (day, month, year), while another may view date as (year, month, day). Some users may view calculated data, which is not actually stored in database. The calculated data is created temporarily when needed. For example, marks of subject "C++" of students are stored in the database and average marks of this subject are calculated by system when the user refers to it. Similarly, if a date-of-birth of a student is stored in the database then you can find the age of the student.
The external views are defined by means of external schemas, which are written in the data definition language (DDL). Usually, the DBA writes an external schema to create a user view. Each user's schema gives a complete description of each type of external record that appears in that user's view. An external record is a record seen by a particular user (a part of his external view). Actually, an external view is a collection of external records.
The external schemas are compiled by the DBMS and stored in its data dictionary. The DBMS uses the external schema created for a specific user, to create a user interface to access the database. Thus user-interface created through external schema accepts and displays information in the format the user expects. It also acts as barrier to hide the information of database from user that are not permitted to him. Typically, it hides the conceptual, internal, and physical details from the user. If any user needs some advance access of database such as to change the existing record of database then the external schema for that user is re-written by DBA to allow access to him.
The external view level is closest to the users. It is concerned with the way the data is viewed by individual users. You can say that external level is the individual user level. A user can either be an application programmer or an end-user, but DBA is an important special case.
The external level consists of many different external views of the database. Each external view describes the part of the database that a particular user group is interested in and hides the rest of the database from that user group. In addition, different views may have different representations of the same data. For example, one user may view date in the form (day, month, year), while another may view date as (year, month, day). Some users may view calculated data, which is not actually stored in database. The calculated data is created temporarily when needed. For example, marks of subject "C++" of students are stored in the database and average marks of this subject are calculated by system when the user refers to it. Similarly, if a date-of-birth of a student is stored in the database then you can find the age of the student.
The external views are defined by means of external schemas, which are written in the data definition language (DDL). Usually, the DBA writes an external schema to create a user view. Each user's schema gives a complete description of each type of external record that appears in that user's view. An external record is a record seen by a particular user (a part of his external view). Actually, an external view is a collection of external records.
The external schemas are compiled by the DBMS and stored in its data dictionary. The DBMS uses the external schema created for a specific user, to create a user interface to access the database. Thus user-interface created through external schema accepts and displays information in the format the user expects. It also acts as barrier to hide the information of database from user that are not permitted to him. Typically, it hides the conceptual, internal, and physical details from the user. If any user needs some advance access of database such as to change the existing record of database then the external schema for that user is re-written by DBA to allow access to him.
