The syntax for enabling and disabling a constraint

To enable a constraint:

ALTER TABLE mytable ENABLE CONSTRAINT mytable_c1;

or, if you wish to enable the constraint and validate its correctness at the same time:

ALTER TABLE mytable MODIFY CONSTRAINT mytable_c1 ENABLE VALIDATE;

Note that the following syntax is invalid and will throw ORA-00933: SQL command not properly ended:

ALTER TABLE mytable ENABLE CONSTRAINT mytable_c1 VALIDATE;

To disable a constraint:

ALTER TABLE mytable DISABLE CONSTRAINT mytable_c1;

or

ALTER TABLE mytable MODIFY CONSTRAINT mytable_c1 DISABLE;