How to make everything null when there is null - sql-server

Question

I want values to be null if there is any null in particular group.
When I am using aggregate functions, ie sum, then it is handling null itself but I don't want to handle null, I need null should make null when doing sum on it.
declare #tbl table (id int,grpid char )
insert into #tbl
values(1,'a'), (2,'b'),(null,'b')
select grpid,sum(id) as val
from #tbl
group by grpid
required output :
grpid val
a 1
b null

Answer 1

Simple as this I guess...
declare #tbl table (id int,grpid char )
insert into #tbl
values(1,'a'), (2,'b'),(null,'b')
;WITH cte AS(
SELECT DISTINCT grpid
FROM #tbl
WHERE id IS NULL
)
select t.grpid,sum(id) as val
from #tbl t
WHERE grpid NOT IN (SELECT grpid FROM cte)
GROUP BY t.grpid
UNION
SELECT grpid, NULL
FROM cte

Answer 2

You can try these methods:
Method 1:
select grpid,
case
when count(*)<>count(id) then null
else sum(id)
end as val
from #tbl
group by grpid
Note: Count(*) always return the number of records but Count(ColumnName) always return the number of records where ColumnName is not null
Method 2:
select grpid,
case
when exists(Select top 1 * from #tbl where grpid = t.grpid and id is null) then null
else sum(id)
end as val
from #tbl as t
group by grpid
Method 3:
select grpid, sum(id) as val
from #tbl as t
where grpid NOT IN (Select grpid from #tbl where id is null)
group by grpid
union
select grpid , null as val from #tbl where id is null

Answer 3

Another approach
declare #tbl table (id int, grpid char );
insert into #tbl
values (1,'a'), (2,'b'), (null,'b'), (3, 'a'), (null, 'c');
select t1.grpid
, case when t2.grpid is null then sum(t1.id)
else null
end as 'ssum'
from #tbl t1
left join #tbl t2
on t2.grpid = t1.grpid
and t2.id is null
group by t1.grpid, t2.grpid
order by t1.grpid;
Similar to some existing but might be more efficient
UNION will perform a distinct
declare #tbl table (id int, grpid char );
insert into #tbl
values (1,'a'), (2,'b'), (null,'b'), (3, 'a'), (null, 'c');
with cte as
( select id, grpid
from #tbl
where id is null
)
select grpid, id
from cte
union
select grpid, sum(id) as val
from #tbl
where not exists (select 1 from cte where cte.grpid = [#tbl].grpid)
group by grpid
order by grpid;