hive聚合函数sum处理null和空字符串时的不同点

create table tmp as 
select  nulll as c1
union all
select null as c1
;
select sum(c1) from tmp
;
=>null

create table tmp2 as 
select  '' as c1
union all
select null as c1
;
select sum(c1) from tmp2
;
=>0
;