两表（多表）关联update 写法

update customers a
   set    city_name=(select b.city_name 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
                 )

   -- update 超过2个值
   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
                 )