count(表达式) :
如果表达式值为NULL则不加1，如果值都为NULL则结果为0
select
count( case when c1 is not null then c1 else null end ) as num
from (
select
null as c1
union ALL
select
null as c1
)t1 -- 结果是0 ,结果不是NULL
select
count( case when c1 is not null then c1 else null end ) as num
from (
select
1 as c1
union ALL
select
null as c1
)t1 -- 结果是1
select count(DISTINCT c1)
from
(
select 2 as c1
union ALL
select 1 as c1
union ALL
select null as c1
)t1 ; -- 2 ,不会包含NULL
end
