怎么更新这个表？多谢了

[jamesyue2008]

a表：
Id Qty
3 150
4 160

b表
Id qty
1 100
2 300
3 Null
4 Null

怎么把a表的数据拷入b表
变b表为：
Id qty
1 100
2 300
3 150
4 160

[老猫五号]

update b
set b.qty = a.qty
from b inner join a where a.id = b.id

[老猫五号]

update b
set b.qty = a.qty
from b inner join a on a.id = b.id

[开着拖拉机泡妞]

update b set qty=qty+a.qty from a where a.id=b.id

[开着拖拉机泡妞]

update b set qty=a.qty from a where a.id=b.id--更正：刚刚看错了

[dawugui]

update b set qty = a.qty from b , a where b.id = a.id

update b set qty = isnull((select qty from a where a.id = b.id),b.qty) from b