Counting null and non-null values in a single query

Question

This works for Oracle and SQL Server (you might be able to get it to work on another RDBMS):

select sum(case when a is null then 1 else 0 end) count_nulls
     , count(a) count_not_nulls 
  from us;

Or:

select count(*) - count(a), count(a) from us;

If I understood correctly you want to count all NULL and all NOT NULL in a column...

If that is correct:

SELECT count(*) FROM us WHERE a IS NULL 
UNION ALL
SELECT count(*) FROM us WHERE a IS NOT NULL

Edited to have the full query, after reading the comments :]

---

SELECT COUNT(*), 'null_tally' AS narrative 
  FROM us 
 WHERE a IS NULL 
UNION
SELECT COUNT(*), 'not_null_tally' AS narrative 
  FROM us 
 WHERE a IS NOT NULL;

Here is a quick and dirty version that works on Oracle :

select sum(case a when null then 1 else 0) "Null values",
       sum(case a when null then 0 else 1) "Non-null values"
from us

for non nulls

select count(a)
from us

for nulls

select count(*)
from us

minus 

select count(a)
from us

Hence

SELECT COUNT(A) NOT_NULLS
FROM US

UNION

SELECT COUNT(*) - COUNT(A) NULLS
FROM US

ought to do the job

Better in that the column titles come out correct.

SELECT COUNT(A) NOT_NULL, COUNT(*) - COUNT(A) NULLS
FROM US

In some testing on my system, it costs a full table scan.

As i understood your query, You just run this script and get Total Null,Total NotNull rows,

select count(*) - count(a) as 'Null', count(a) as 'Not Null' from us;

usually i use this trick

select sum(case when a is null then 0 else 1 end) as count_notnull,
       sum(case when a is null then 1 else 0 end) as count_null
from tab
group by a

Just to provide yet another alternative, Postgres 9.4+ allows applying a FILTER to aggregates:

SELECT
  COUNT(*) FILTER (WHERE a IS NULL) count_nulls,
  COUNT(*) FILTER (WHERE a IS NOT NULL) count_not_nulls
FROM us;

SQLFiddle: http://sqlfiddle.com/#!17/80a24/5