如果要更新数据库中的表，可以执行以下操作：

create table newtable
   (`cartid` int, `prodid` int unique key, `quantity` int);

insert into newtable
   select * from yourtable order by cartid
   on duplicate key update quantity=newtable.quantity+values(quantity)

select * from newtable

输出：

cartid  prodid      quantity
1       9226582     8
3       7392588     1

如果你对结果满意，你可以

drop table yourtable
alter table newtable rename to yourtable

使用group by和min-
看看这个-http://sqlfiddle.com/#!9/6c4332/4号

select min(cartid) cartid ,prodid,sum(quantity) quantity
from 
yourtable
group by prodid
order by cartid

创建另一个具有相同架构的表，然后

insert into newtable 
select min(cartid) cartid ,prodid,sum(quantity) quantity
    from 
    yourtable
    group by prodid
    order by cartid

重命名newtable