Database Reference
In-Depth Information
Entity Integrity Rule:
The entity integrity rule states that no component of the
primary key in a base relation is allowed to accept nulls. Put another way, in a relational
model, we never record information about something that we cannot identify. Three
points are worth noting here:
•
The rule applies to base relations.
•
The rule applies to primary key, not alternate keys.
•
The primary key must be wholly non-null.
Referential Integrity Rule:
The referential integrity rule states that the database
must not contain unmatched foreign key values. By unmatched foreign key value, we
mean a non-null foreign key value for which there is no match in the referenced (target)
relation. Put another way, if B references A then A must exist. The following points should
be noted:
•
The rule requires that foreign keys must match primary keys, not
alternate keys.
•
Foreign key and referential integrity are defined in terms of
each other. It is not possible to explain one without mentioning
the other.
4.2 Foreign Key Concept
The concept of a foreign key was introduced in the previous chapter. Let us revisit this
concept, by introducing a more formal definition:
We will use the notation R1 → R2 to mean, the relation R1 references the relation R2.
In this case, R1 is the referencing (primary) relation and R2 is the referenced relation.
Since R1 is the referencing relation, it contains a foreign key. We will also use the notion
R1{A, B, C, ...} to mean, the relation R1 contains attributes A, B, C, and so on. Where
specific examples are given, the relation-name will be highlighted or placed in upper
case; attribute-names of specific examples will not be highlighted when stated with the
related relation; however, they will be highlighted when reference is made to them from
the body of the text.