Database management system provide a facility known as data definition language (DDL), which can be used to define the conceptual scheme and also give some details about how to implement this scheme in the physical devices used to store the data.
This definition includes:
- All entity sets and their associated attribute as well as the relationships among the entity sets.
- Any constraints that have to be maintained, including the constraints on the value that can be assigned to a given attribute and the constraints on the value assigned to different attributes in the same or different records.
These definitions, which can be described as meta data about the data in the database, are expressed in the DDL of the DBMS and maintained in a compiled form (usually as a set of tables). The compiled form of the definition is known as a data dictionary, directory, or system catalog. The data dictionary contains information on the data stored in the database and is consulted by the DBMS before any data manipulation operation.
Thus information pertaining to the structure and usage of data contained in the database, the metadata, is maintained in the data dictionary. The data dictionary is the database itself, documents the data. Each database user can consult the data dictionary to learn what each piece of data and the various synonyms of the data fields mean.
In an integrated system (i.e., in a system where the data dictionary is a part of the DBMS) the data dictionary stores information concerning the external, conceptual, and internal levels of the database. It contains the source of each data-field value, the frequency of its use, and an audit trail concerning updates, including the "who" and "when" for each update.
Currently data dictionary systems are available as add-on to the DBMS. Standards have yet to be evolved for integrating the data dictionary facility with the DBMS so that the two databases, one for metadata and the other for data, can be manipulated using a unified DDL/DML.
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