with cte1
as(
select 1 as id union all
select 2 union all
select null union all
select 3 union all
select null
),
cte2
as(
select count(id) as cnt_notnull,count(case when id is null then 0 else null end) as cnt_null
from cte1 
)
select *,case when cnt_notnull = 0 then 0 else cnt_null/cnt_notnull *100 end as rate from cte2

cnt_notnull cnt_null rate
3 2 66.6667