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A data dictionary is a file or a set of files that contains a database's metadata. The data dictionary contains records about other objects in the database, such as data ownership, data relationships to other objects, and other data.
The data dictionary is a crucial component of any relational database. Ironically, because of its importance, it is invisible to most database users. Typically, only database administrators interact with the data dictionary.
In a relational database, the metadata in the data dictionary includes the following:
For most relational database management systems (RDBMS), the database management system software needs the data dictionary to access the data within a database. For example, the Oracle DB software has to read and write to an Oracle DB. However, it can only do this via the data dictionary created for that particular database.
For instance, suppose that in a commercial bank's database, the administrator wants to determine which table holds information about loans. Making an educated guess that the table most likely has the word "LOAN" in it, he would issue the following query on the data dictionary (the first query is for an Oracle DB, while the second is for an SQL Server DB):