ALTER TABLE employees
  ADD COLUMN user_id INT NULL AFTER desg_id,
  ADD CONSTRAINT fk_employees_user FOREIGN KEY (user_id) REFERENCES users (id) ON DELETE SET NULL,
  ADD UNIQUE KEY uq_employees_user (user_id);

ALTER TABLE users
  ADD CONSTRAINT fk_users_employee FOREIGN KEY (employee_id) REFERENCES employees (id) ON DELETE SET NULL;