THE RELATIONAL DATABASE HIERARCHY

A relational database is organized in a hierarchical structure, where the highest level is the catalog. Generally only the largest, most complex databases have multiple catalogs.

• Catalogs: A database catalog comes into play only in large, complex databases that have multiple schemas.

• Schemas: A database schema contains metadata. This metadata includes defini- SQL and the Relational Model tions of tables, views, value ranges, indexes, users, and user groups. It can also include stored procedures and triggers.

• Tables: A database table is a set of elements organized as a two-dimensional table with horizontal rows and vertical columns. The columns correspond to the attri- butes in the ER model of an entity. The rows hold the data about individual instances of the entity.

• Columns: A column is a component of a database table. Each column in the table corresponds to one of the attributes in the ER model of the entity being actualized by the table.