Data Dictionary is a repository of information about data in a database or a data set (a type of metadata).

Active data dictionary is a data dictionary managed by DBMS. Every change in database structure (using DDL - Data Definition Language) is automatically reflected in the data dictionary.

Accessing data dictionary

Most relational databases provide read access to its active data dictionary with a predefined set of read only tables or views that you can query to get access to database metadata - list of tables, columns, relationships etc.

Different vendors use different names for its data dictionary - system catalog, catalog tables, data dictionary, information schema, but the idea is almost always the same.

Here is a big database of useful queries to active data dictionary of various DBMS.

Learn more

1. What is Data Dictionary
2. What is Passive Data Dictionary (and what are the benefits)
3. What is Information_schema?
4. System Catalog / Data Dictionary Views