Databases change over time as information is inserted and deleted. 'The collection of information stored in the database at a particular moment is called an instance of the database. The overall design of the database is called the database schema. If schema change at all, they change very infrequently.
A database schema corresponds to the programming-language type definition. A variable of a given type has a particular values at a given instant. Thus, the values of a variable in programming languages correspond to an instance of a database scheme.
Database systems have general schema, partitioned according to the levels of abstraction. At the lowest level is the physical schema, at the intermediate level is the logical schema, and at the highest level is a sub-schema. In general, database systems' support one physical schema, one logical schema and several sub-schema.
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