SQL error 'Warning: Null value is eliminated by an aggregate or other SET operation.'

    警告:null值被一个聚合或其它设置操作清除。    

create table test
(
id int,
cnt int
)

insert test
select 1,null
union 
select 1,2
union
select 2,3

select id,sum(cnt) from test group by id

    聚合函数无法对null值进行运算，所以null值会被忽略掉。