在spark的空表test上进行运算，注意结果差异：
select name,sum(if(age is null,0,age)) as ages from test group by name;//空
select sum(if(age is null,0,age)) as ages from test group by name;//空
select sum(if(age is null,0,age)) as ages from test; //null
原文：http://www.cnblogs.com/yongwangzhiqian/p/5772497.html