Specifies a constraint that column values inserted into the table
    must satisfy. You can specify more than one column constraint.
    For example:

    SQL> ALTER TABLE EMPLOYEE
    cont>   ADD ID_NUMBER INT NOT NULL UNIQUE;

    You can name each constraint. For example:

    SQL> ALTER TABLE EMPLOYEE
    cont>   ADD ID_NUMBER INT
    cont>   CONSTRAINT A NOT NULL
    cont>   CONSTRAINT B UNIQUE;