FOR UPDATE OF

Allows you to lock columns of a table while a query is performed. Use the NOWAIT keyword to return immediately with a notification if unable to update due to a lock, rather than waiting for the lock to be removed.

Example:

DECLARE
    CURSOR cur_move_emp(p_emp_loc employee.emp_loc%TYPE) IS
        SELECT emp_id, dept_name
            FROM employee, departments
            WHERE emp_dept_id = dept_id
                AND emp_loc = p_emp_loc
            FOR UPDATE OF emp_loc NOWAIT;  -- lock emp_loc column while we update,
BEGIN                                      -- and return immediately if already locked
    FOR cur_move_emp_var IN cur_move_emp('CA') LOOP
        UPDATE employee
            SET emp_loc = 'WA'
        WHERE CURRENT OF cur_move_emp;  -- using where current of instead of where <condition>
    END LOOP;
    COMMIT;  -- remove the lock
END;