Constraints in databases are used to define an integrity
constraint, as a rule that restricts the values in a data base.
Now we are discussing 5 types of constraints…
Ø NOT NULL Constraint
Ø UNIQUE Constraint
Ø PRIMARY KEY Constraint
Ø FOREIGN KEY Constraint
Ø CHECK Constraint
Declaration Style:
Ø Column level (OR) IN LINE Style.
Ø Table level (OR) OUT OF LINE Style.
Column Level:
Ø They are declared as part of the
definition of an individual column or attribute.
Ø Usually applied when the constraint
is specific to that column only.
Table Level:
Ø They are declared as part of the
table definition.
Ø Definitely applied when the
constraint is applied on combination of columns together.
Note:
Ø NOT
NULL Constraint is the only constraint which should be declared as inline only.
Ø Every constraint is managed by
oracle with a constraint name in the Meta data.
Ø Hence when we declare a constraint
if we do not provide a constraint name oracle associates the constraint with
name.
Ø Within a single user no two
constraints can have the same name.
Ø Rather than depending on the oracle
supplied constraint name, it is better to define our own name for all
constraints.
Ø When constraints are named we should
use ‘CONSTRAINT’ clause.
The CONSTRAINT clause can appear in:
Ø Create and Alter table statement.
Ø Create and Alter view statement.
Ø Oracle does not support constraints
on columns or attributes whose data type is
o
USER_DEFINED
OBJECTS.
o
NESTED
TABLES and VARRAY
o
REF
and LOB.