Explain the term database schema and instance.Ripunjay Tiwari
Ans. Schema – Schema is the logical description of entire database. The overall design of the database is called database schema and it is changed rarely.
Database systems have several schemas, partitioned according to the levels of abstraction.
At lowest level is physical schema, at intermediate level is logical schema and at highest level is subschema or external schema. Generally, database systems support one physical schema and several subschemas.
Basically, internal schema describes how the data are actually stored on disk. Conceptual schema describes the structure of the database to database designer. External schema or subschema describes the structure of database to end users. An end user gets the view of the database via external schema.
Instance – Generally, the database is changed over time, as data inserted, deleted or updated. The state of database at any particular moment of time i.e., the collection of data and information along with various objects definition is called the instance of the database.
In database terminology, the structure is a schema and the instance of schema is a database.