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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
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原文地址：http://www.cnblogs.com/yongwangzhiqian/p/5772497.html
