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Analysis and Design
Integrity Rules in Relational Database Model
Integrity rule 1: Entity integrity
It says that no component of a primary key may be null.
All entities must be distinguishable. That is, they must have a unique identification
of some kind. Primary keys perform unique identification function in a relational
database. An identifier that was wholly null would be a contradiction in terms.
It would be like there was some entity that did not have any unique identification.
That is, it was not distinguishable from other entities. If two entities are not
distinguishable from each other, then by definition there are not two entities
but only one.
Integrity rule 2: Referential integrity
The referential integrity constraint is specified between two relations and
is used to maintain the consistency among tuples of the two relations.
Suppose we wish to ensure that value that appears in one relation for a given
set of attributes also appears for a certain set of attributes in another. This
is referential integrity.
The referential integrity constraint states that, a tuple in one relation that
refers to another relation must refer to the existing tuple in that relation.
This means that the referential integrity is a constraint specified on more than
one relation. This ensures that the consistency is maintained across the relations.
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