Data Dictionary:
A relational database system have to maintain data about the tables and all other objects in the database. This information is known as the data dictionary or the system catalog. A data dictionary is a file that describes the way in which the database is organized. Data dictionaries do not has any real data from the database but only bookkeeping information about how to manage the data. It contains details of all the files in the database, the records in every file, and the details of the fields in each record.
The data dictionary have to store names of the relations and the attributes of each relation, the domains and lengths of attributes, the names of views described in the database, the definitions of those views, and the integrity or key constraints. A view is a virtual table that comprises the fields of one or more tables in the database or fields from other views. The reason a view is known as a virtual table is because it does not store or contain data. It draws its data from the tables or views upon which it is based. Because a view does not save data, only its structure is saved in the database, and it is recreated each time it is accessed.
Data dictionaries are used by the database administrator to query the status of a firm's data. The administrator may create changes to the definitions of selected data to incorporate new organizational requirements. Data dictionaries enforce standard data element definitions whenever end users and application programs use a DBMS to access an organization's databases. For example, an active data dictionary would not allow a data entry operator to enter a non-standard definition of a customer record, nor would it allow him/her to enter a name that exceeds the defined size of that data element.
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