Data Dictionary Types
A data dictionary holds the information about the logical structure of the database. Here is where all the descriptions of record types, data of item types, and data aggregates in the database. Also called a database about the database.
A data dictionary can either be part of the DMBS or can be available without a DMBS. When the data dictionary is part of the DMBS it is called an integrated data dictionary. It is always consistent with the actual database structure, due to it being maintained automatically by the system. When the data dictionary is available without a DMBS it is called a freestanding data dictionary. This type of data dictionary can be a commercial product or a simple file developed and maintained by the designer.
Freestanding Data Dictionary
Both types of data dictionaries have their pros and cons. However the use of the freestanding data dictionary is the most useful. It allows the designer to design without the commitments to the particular implementation. A big disadvantage is that once the database has been created, all adjustments to its structure may not be entered into the freestanding data dictionary and, eventually, the data dictionary will not be a correct reflection of the database structure. A freestanding data dictionary is useful for the many things. Collection and storing information about data in a central location, Securing agreement from users and designers about the meanings of data items, communicating with users, identifying redundancy and inconsistency in data item names, keeping track of changes to the database structure, determining the impact of changes to the database structure, identifying the sources of and responsibility for the correctness of each item, recording the external, logical, and internal schemas and the mapping between them, recording access control information, and last, a DMBS data dictionary can also be useful for providing audit information.