Using SUM so that NULL in columns make sum NULL

Question

I have a a table with a column groups INTEGER NULL. It has values

groups

5
7
<NULL>

If I do a select sum(groups) form table_name
I would get 12. How can I get null, when the column being summed has a null.

Answer 1

One option:

CASE WHEN COUNT(*) = COUNT(groups) THEN SUM(groups) ELSE NULL END

Answer 2

select
  case when exists (select groups from table where groups is null) then null
       else select sum(groups) from table
  end as grp_sum