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
???????????????? )
