ADD COLUMN `hora_emissao`  time NOT NULL DEFAULT CURRENT_TIME;

DELIMITER $$

CREATE
    TRIGGER horaatual BEFORE INSERT ON TIME
    FOR EACH ROW BEGIN
       SET NEW.hora=CURTIME();
    END$$

DELIMITER ;

CREATE TABLE `suaTabela` (
  `datahora` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
)

SELECT DATE_FORMAT(datahora,"%H:%i:%s") AS hora FROM suaTabela