Oracle 数据库 根据B表的数据去更新A表

update A

set A.字段2 = (select B.id from B where A.字段2 = B.字段2)

where exists (select 1 from B where B.字段2 = A.字段2);

-- 如果B表字段2有重复，那么会报错：单条语句返回多行。此时用下面语句：
update A

set A.字段2 = (select B.id from B where A.字段2 = B.字段2 and rownum=1)

where exists (select 1 from B where B.字段2 = A.字段2);