---2.
create table t1
(
    id int,
    com int
)
create table t2
(
    id int,
    price int
)

insert into t1 select 1,50 union all
select 2,60 union all
select 1,30 union all
select 1,40 union all
select 2,80 

insert into t2 select 1,NULL union all
select 2,NULL
----------------------------
update t2 set price=s from t2,(select id,sum(com) s from t1 group by id)tmp where t2.id=tmp.id

------解决方案--------------------

探讨

SQL code


--问题1：
相当于：cross jion？