SCHEMA-DAY5

Instances and Schemes

The data stored in the database at any given time is an instance of the database.

The overall design of the database is the database scheme.

Example:

Name	account number	balance
TOM CRUISE	1024	8725.38
DANY WALT	4977	10.50

Is an instance of a database with scheme (name, account number, balance)

Database systems have schemes (designs) at each level of abstraction:

· The physical scheme is the design at the physical level, i.e. as a file of records of a particular type

· The conceptual scheme is the design at the conceptual level. Example: (name, account number, balance) is a conceptual scheme.

· A subscheme is a design at the view level

· A sub scheme is part of a scheme

· Example: (name, account number) is a sub scheme of (name, account number, balance)

· The sub scheme controls what information can be seen using the corresponding view

· Example: the previous sub scheme could be used by someone preparing ATM cards for the bank

As with levels of abstraction, there is:

· One physical scheme

· One conceptual scheme

As many sub schemes as needed