关于两表关联的update

   -- update 一个字段

   update a
   set a.value1 = (select b.value2 from b where b.id2 = a.id1)
where exists (select 1 from b where b.id2 = a.id1)

   -- update 超过2个字段
   update customers a 
   set    (city_name,customer_type)=(select b.city_name,b.customer_type
                                     from   tmp_cust_city b
                                     where  b.customer_id=a.customer_id)
   where  exists (select 1
                  from   tmp_cust_city b
                  where  b.customer_id=a.customer_id
                 )

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