The data dictionary is the foundation of the database--it's where the RDBMS finds which tables are in a database, which columns are in the tables, which columns are primary or foreign keys, and what type of data to expect in those columns. If you modify a table, such as by adding a column or constraint, the RDBMS automatically updates the data dictionary to reflect the change.
Formats for data dictionaries differ among RDBMSs, but they usually contain:
- Definitions of table columns
- Referential-integrity constraints among tables
- Definitions of views and custom data domains