Counting null and non-null values in a single query

Question

I have a table

create table us
(
 a number
);

Now I have data like:

a
1
2
3
4
null
null
null
8
9

Now I need

Answer 1

Try

SELECT 
   SUM(ISNULL(a)) AS all_null,
   SUM(!ISNULL(a)) AS all_not_null
FROM us;

Simple!