在以下业务场景中，我们有一张订单表，订单表中有产品的编号，我们需要匹配出产品所属的品牌名称。不同品牌的产品名称分别存放在不同的产品清单表中。

在MySQL等关系型数据库中，可以使用case when+in(子查询)的方式来匹配订单表中的产品所属的品牌名称。

select product_code
	,product_quantity
	,case when product_code in (select product_code from table_a)
		  then 'BRAND_A'
		  when product_code in (select product_code from table_b)
		  then 'BRAND_B'
		  when product_code in (select product_code from table_c)
		  then 'BRAND_C'
		  else null
	  end as brand_name
from 
table_order

但是在HIVE SQL中，如果执行该查询语句，就会报错，提示Currently SubQuery expressions are only allowed as Where Clause predicates。
这个时候可以使用left join来解决。

with tmp1 as
(
select product_code
	,product_quantity
	,case when t2.product_code is not null then 'BRAND_A'
	      else null
	 end as brand_name
from
table_order t1
left join table_a t2
on t1.product_code=t2.product_code
)
,tmp2 as 
(
select product_code
	,product_quantity
	,case when t2.product_code is not null then 'BRAND_B'
	      else t1.brand_name
	 end as brand_name
from
tmp1 t1
left join table_b t2
on t1.product_code=t2.product_code
)
,tmp3 as 
(
select product_code
	,product_quantity
	,case when t2.product_code is not null then 'BRAND_C'
	      else t1.brand_name
	 end as brand_name
from
tmp2 t1
left join table_c t2
on t1.product_code=t2.product_code
)
select product_code,product_quantity,brand_name from tmp3;