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

Number of elements where a is null:

select count(a) from us where a is null;

Number of elements where a is not null:

select count(a) from us where a is not null;