Similar to types and variables in programming languages, there are Instances and Schemas in Database Systems.

Schema– the logical structure of the database

• Example: The database consists of information about a set of customers and accounts and the relationship between them).
• Analogous to type information of a variable in a program.
• Physical schema: database design at the physical level.
• Logical schema: database design at the logical level.

Instance– the actual content of the database at a particular point in time

• Analogous to the current values of a set of variables.

Physical Data Independence– the ability to modify the physical schema without changing the logical schema

• Applications depend on the logical schema (or the view level schema)
• In general, the interfaces between the various levels and components should be well defined so that changes in some parts do not seriously influence others.