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 consist of many different external views of database. Each external view describes 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 schemes, which are written in the data definition language (DDL). Usually, the DBA writes an external schema to create a user view. Each 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 schemes are compiled by the DBMS and store 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 the user that are not permitted to him.
The external level consist of many different external views of database. Each external view describes 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 schemes, which are written in the data definition language (DDL). Usually, the DBA writes an external schema to create a user view. Each 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 schemes are compiled by the DBMS and store 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 the user that are not permitted to him.
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