50
2.2.4.1.4 Keys
1. Super key
Super Key is an attribute or set of attributes that uniquely
identifies a tuple within a relation.
2. Candidate Key
Minimal set of attributes
that
uniquely
identifies each
occurrence of an entity type.
Super
key
(K)
such
that
no
proper
subset
is
a
super
key within the relation.
In each tuple of R,
values of K
uniquely
identify that
tuple (uniqueness).
No
proper
subset
of
K
has
the
uniqueness
property
(irreducibility).
3. Primary Key
Primary
Key
is
a
candidate
key
selected
to
uniquely
identify each occurrence of an entity type.
4. Foreign key
Foreign Key is an attribute, or set of attributes, within one
relation that matches candidate key of some (possibly
same) relation.
 |