oracle 数据库中修改表中某字段的值等于另一个表中字段的值

模板：

UPDATE t2 SET t2.C_date = (select EventTime from t1 where t1.id = t2.id) 
where exists (select 1 from t1 where t1.id = t2.id)

实际使用：

UPDATE hc_fee_order_item it SET it.fee_charge =
(
select fo.amount_discounted from hc_fee_order fo 
where fo.id = it.fee_order_id and fo.order_name = '流量会员服务'
)
where exists
(
select 1 from hc_fee_order fo 
where fo.id = it.fee_order_id and fo.order_name = '流量会员服务'
);