A deferred constraint is one that is enforced when a transaction is committed.
A deferrable constraint is specified by using DEFERRABLE clause.
Once you've added a constraint, you cannot change it to DEFERRABLE. You must drop and recreate the constraint.
When you add a DEFERRABLE constraint, you can mark it as INITIALLY IMMEDIATE or INITIALLY DEFERRED.
INITIALLY IMMEDIATE means that the constraint is checked whenever you add, update, or delete rows from a table.
INITIALLY DEFERRED means that the constraint is only checked when a transaction is committed.
SQL> -- create demo table
SQL> create table myTable(
2 id NUMBER(2),
3 value NUMBER(6,2)
4 )
5 /
Table created.
SQL>
SQL> -- prepare data
SQL> insert into myTable(ID, value)values (1,9)
2 /
1 row created.
SQL> insert into myTable(ID, value)values (2,2.11)
2 /
1 row created.
SQL> insert into myTable(ID, value)values (3,3.44)
2 /
1 row created.
SQL> insert into myTable(ID, value)values (4,-4.21)
2 /
1 row created.
SQL> insert into myTable(ID, value)values (5,10)
2 /
1 row created.
SQL> insert into myTable(ID, value)values (6,3)
2 /
1 row created.
SQL> insert into myTable(ID, value)values (7,-5.88)
2 /
1 row created.
SQL> insert into myTable(ID, value)values (8,123.45)
2 /
1 row created.
SQL> insert into myTable(ID, value)values (9,98.23)
2 /
1 row created.
SQL>
SQL> select * from myTable
2 /
ID VALUE
---------- ----------
1 9
2 2.11
3 3.44
4 -4.21
5 10
6 3
7 -5.88
8 123.45
9 98.23
9 rows selected.
SQL>
SQL> ALTER TABLE myTable
2 ADD CONSTRAINT uq UNIQUE (id)
3 DEFERRABLE INITIALLY DEFERRED;
Table altered.
SQL>
SQL> -- clean the table
SQL> drop table myTable
2 /
Table dropped.
SQL>