Need a SQL select statement to return rows that have the same id in one column and distinct value in another column

Question

I have a table that contains a group number column and a data column:

GROUP	DataColumn
1	NULL
1	NULL
1	"hello"
1	NULL
2	"bye"
2	"sorry"
3	NULL
3	NULL
3	NULL

I want to return the string in the DataColunm as long as all rows in that group contain a string (no row is null).

If any row in the group is NULL then I'd like to return all rows in that group with NULL in the DataColumn .

My desired output would be:

GROUP	DataColumn
1	NULL
1	NULL
1	NULL (swap "hello" to null since the other values for group 1 are null)
1	NULL
2	"bye"
2	"sorry"
3	NULL
3	NULL
3	NULL

Answer 1

Here's one option: check whether number of null and not null values per each group is a positive number; if so, return null for that group.

Sample data:

SQL> set null NULL
SQL> with test (cgroup, datacolumn) as
  2    (select 1, null    from dual union all
  3     select 1, null    from dual union all
  4     select 1, 'hello' from dual union all
  5     select 1, null    from dual union all
  6     select 2, 'bye'   from dual union all
  7     select 2, 'sorry' from dual union all
  8     select 3, null    from dual union all
  9     select 3, null    from dual union all
 10     select 3, null    from dual
 11    ),

Query begins here:

 12  temp as
 13    (select cgroup, datacolumn,
 14       sum(case when datacolumn is null then 1 else 0 end) over (partition by cgroup) cnt_null,
 15       sum(case when datacolumn is null then 0 else 1 end) over (partition by cgroup) cnt_not_null
 16     from test
 17    )
 18  select cgroup,
 19    case when cnt_null > 0 and cnt_not_null > 0 then null
 20         else datacolumn
 21    end as datacolumn
 22  from temp;

    CGROUP DATACOLUMN
---------- ---------------
         1 NULL
         1 NULL
         1 NULL
         1 NULL
         2 bye
         2 sorry
         3 NULL
         3 NULL
         3 NULL

9 rows selected.

SQL>

Answer 2

Use COUNT() window function to count all the rows of each GROUP and compare the result to the number of the rows with non-null values:

SELECT "GROUP",
       CASE 
         WHEN COUNT(*) OVER (PARTITION BY "GROUP") = 
              COUNT("DataColumn") OVER (PARTITION BY "GROUP") 
            THEN "DataColumn" 
       END "DataColumn"
FROM tablename;

See the demo .