MySQL存储过程实现对账单，如何使用游标

需要开发一个存储过程 ，从 sellorder 表
create table sellorder (
id int primary key auto_increment,
price decimal,
amount int
)
中，按 id 从小到大加载订单数据，生成对账单，写入 bill表
create table bill(
id int primary key auto_increment,
total decimal
)
其中 bill 的 total字段保存了交易单的总金额，这个金额是每一张对账单的所有 sellorder 的 amount 总和，其数值不会超过 20000。

create procedure make_bill()
begin
    DECLARE done INT DEFAULT FALSE;
    declare cur_sellorder  cursor for select id, ,amount from sellorder  order by id limit 1000;
    declare order_id, amount INT;    
    DECLARE CONTINUE HANDLER FOR NOT FOUND SET done = TRUE;    
    open cur_sellorder ;
    bill: LOOP
        fetch cur_sellorder  into order_id, amount;
        if done then
            LEAVE bill;
        end if;
    if @total + @amount > 20000 then
        leave bill;
    else     
    end if;
    END LOOP bill;
    CLOSE cur_sellorder ;
    start transaction;
    delete from sellorder  where id < order_id;
    commit ;
end;