instances and schemas

• Similar to type and variable in programing languages .
• Schema - the logoical structure of the Database 

            -E.X the database consists of information about a set of customers and account 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 value of a Variable.
• Physical Data Independence - the ability to modify the physical schema without changing the logical schema 

1.  Application depend on the logical schema 
2. In general , the interface between the various levels and components should be well defined so that changes in some parts do not seriously influence  others.