Conceptual Schema

Internal Schema

Logical Scheme

External Schema

Attribute

Table

Record

Tuple

field key.

referential integrity.

foreign key.

candidate key.

A join can always be used as an alternative to a subquery, and a subquery can always be used as an alternative to a join.

A join can sometimes be used as an alternative to a subquery, and a subquery can sometimes be used as an alternative to a join.

A join can always be used as an alternative to a subquery, and a subquery can sometimes be used as an alternative to a join.

A join can sometimes be used as an alternative to a subquery, and a subquery can always be used as an alternative to a join.

The range of values that the attribute is allowed to take.

The type of the attribute.

Which tables the attribute is allowed to be in.

More than one of the above.

memo

text

data

long

logically

physically

not

badly

description

attribute

byte

logical element

must be different from foreign keys in another table

can be the same value as other primary key values in the same table

must not contain NULL

must be different from a primary key in another table

Redundant data

Program-data independence

Better data quality

Reduced program maintenance