mysql-触发器处理表字段的唯一性

delimiter $$
DROP TRIGGER IF EXISTS tri_nis_billhead_before_insert $$
CREATE TRIGGER tri_nis_billhead_before_insert 
BEFORE INSERT ON nis_billhead
FOR EACH ROW
BEGIN
        declare counts int;
        select count(1) into counts from nis_billhead where out_sys_bill_no=new.out_sys_bill_no;
        if counts >0 then 
                SIGNAL SQLSTATE 'HY000' SET MESSAGE_TEXT='外部系统单号重复';
        end if;
                
END $$
delimiter ;