Oracle 11g Release 2

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CREATE TABLE tablename ("GROUP" int, "DataColumn" varchar2(7));
INSERT ALL 
    INTO tablename ("GROUP", "DataColumn")
         VALUES (1, NULL)
    INTO tablename ("GROUP", "DataColumn")
         VALUES (1, NULL)
    INTO tablename ("GROUP", "DataColumn")
         VALUES (1, 'hello')
    INTO tablename ("GROUP", "DataColumn")
         VALUES (1, NULL)
    INTO tablename ("GROUP", "DataColumn")
         VALUES (2, 'bye')
    INTO tablename ("GROUP", "DataColumn")
         VALUES (2, 'sorry')
    INTO tablename ("GROUP", "DataColumn")
         VALUES (3, NULL)
    INTO tablename ("GROUP", "DataColumn")
         VALUES (3, NULL)
    INTO tablename ("GROUP", "DataColumn")
         VALUES (3, NULL)
SELECT * FROM dual
;
9 rows affected

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


GROUP
DataColumn


1
null

1
null

1
null

1
null

2
bye

2
sorry

3
null

3
null

3
null

GROUP	DataColumn
1	null
1	null
1	null
1	null
2	bye
2	sorry
3	null
3	null
3	null