Instances and Schemas

Instances and schemas are analogous to variables and types (respectively) in programming languages.

Schema

A schema is the logical structure of the database.

• A physical schema describes the database design at the physical level

• A logical schema describes the database design at the logical level

The ability to modify the physical schema without changing the logical schema is called physical data independence. This is possible because applications only depend on the logical schema, and there are well-defined interfaces between the levels to prevent the changes made in one layer from having a significant impact on the other layers.

Instance

An instance is the actual content of the database at a given point in time.