Oracle多表连接更新

两个表A,B通过字段ID INNER JOIN连接，现想更新表B中的QTY值为表A的QTY值，但只更新这两个QTY不一样的部分，一样的不动。

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晕, 一样的动不动有什么区别吗?

方式1, update

update a
set a.qty = (select qty from b where a.id = b.id)
where exists (select 1 from b where a.id = b.id and a.qty <> b.qty)

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方式2, merge into

merge into a
using (select b.* from  a, b where a.id = b.id and a.qty <> b.qty ) b
on (a.id = b.id)
when matched then
update set a.qty = b.qty
when not matched then
  insert (a.id,a.qty)
  values(b.id,b.qty)

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因为数据量太大，虽然没区别，但这样不是等于整个表更新了吗？

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SQL code:

merge into B
using A
on(B.id = a.id)
when matched then
update set B.QTY=A.QTY where B.QTY<>A.QTY;

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SQL code:
update a
set a.qty = (select qty from b where a.id = b.id)
where exists (select 1 from b where a.id = b.id and a.qty <> b.qty)

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2楼 4楼哪个对？

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都可以。一个是没有找到的情况下就为insert一条建议2楼

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2楼很对啊

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merge into B
using A
on(B.id = a.id)
when matched then
  update set B.QTY=A.QTY where