Question

I am working on a query that is fairly similar the following:

CREATE TABLE #test (a char(1), b char(1))

INSERT INTO #test(a,b) VALUES 
('A',NULL),
('A','B'),
('B',NULL),
('B',NULL)

SELECT DISTINCT a,b FROM #test

DROP TABLE #test

The result is, unsurprisingly,

a   b
-------
A   NULL
A   B
B   NULL

The output I would like to see in actuality is:

a   b
-------
A   B
B   NULL

That is, if a column has a value in some records but not in others, I want to throw out the row with NULL for that column. However, if a column has a NULL value for all records, I want to preserve that NULL.

What’s the simplest/most elegant way to do this in a single query?

I have a feeling that this would be simple if I weren’t exhausted on a Friday afternoon.

Answer 1

Try this:

select distinct * from test
where b is not null or a in (
  select a from test
  group by a
  having max(b) is null)

You can get the fiddle here.

Note if you can only have one non-null value in b, this can be simplified to:

select a, max(b) from test
group by a