A cluster is an SQL session accessing a uniquely named set of catalogs. It is equivalent to an installation of a vendor’s RDBMS system.

A catalog is a uniquely named set of schemas. For very large database systems, such as distributed databases, you may encounter duplicate schemas and a catalog provides a higher hierarchy for grouping.

The schema is a uniquely named set of objects, such as tables, views, system tables, and stored procedures. The system tables contain the self-defining data for describing all of the other objects. The objects and data are owned by a given user. The owner (or creator) then determines who has access privileges to the data and objects.

The tables and views contain columns and rows of data.

You can qualify an object name as follows: CATALOG_NAME.SCHEMA_NAME.TABLE_NAME

> With most RDBMS systems, the cluster controls who gets access to the data.

> With Oracle and SQL Server, the catalog controls who gets access to the data.
For Oracle and SQL Server, you can think of the catalog as equivalent to an instance.