mysql create serial number

mysql database to create serial number

CREATE TRIGGER saledetail_id BEFORE INSERT ON saledetail
FOR EACH ROW BEGIN
declare n int;
select IFNULL(max(right(ItemID,4)),0) into n from saledetail where mid(ItemID,1,8)=DATE_FORMAT(CURDATE(),'%Y%m%d');
set NEW.ItemID=concat(DATE_FORMAT(CURDATE(),'%Y%m%d'),right(10001+n,4));
END;

// Note that when the primary key is inserted to set a default value to insert into it, here I set the empty string ""