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

I created the table in postgres 10 and both of the following worked:

select count(*) from us

and

select count(a is null) from us