mysql custom autonumber

CREATE TABLE _sequence
(
    seq_name VARCHAR(50) NOT NULL PRIMARY KEY,
    seq_group VARCHAR(10) NOT NULL,
    seq_val INT UNSIGNED NOT NULL
);

delimiter //
DROP FUNCTION IF EXISTS getNextCustomSeq//
CREATE FUNCTION getNextCustomSeq
(
    sSeqName VARCHAR(50),
    sSeqGroup VARCHAR(10)
) RETURNS VARCHAR(20)
BEGIN
    DECLARE nLast_val INT;
    SET nLast_val =  (SELECT seq_val
                          FROM _sequence
                          WHERE seq_name = sSeqName
                                AND seq_group = sSeqGroup);
    IF nLast_val IS NULL THEN
        SET nLast_val = 1;
        INSERT INTO _sequence (seq_name,seq_group,seq_val)
        VALUES (sSeqName,sSeqGroup,nLast_Val);
    ELSE
        SET nLast_val = nLast_val + 1;
        UPDATE _sequence SET seq_val = nLast_val
        WHERE seq_name = sSeqName AND seq_group = sSeqGroup;
    END IF;
    SET @ret = (SELECT concat(sSeqGroup,'-',nLast_val));
    RETURN @ret;
END//
delimiter ;


DROP TRIGGER IF EXISTS tr_custom_autonums//
CREATE TRIGGER tr_custom_autonums BEFORE INSERT ON tb_pegawai
FOR each ROW
BEGIN
   SET NEW.id_pegawai = getNextCustomSeq("id_pegawai","PG");
END//
delimiter ;