create table t1 ( key number, group_key varchar2(10), val varchar2(10) ); insert into t1 values ( 1, 'Group-1', null ); insert into t1 values ( 2, 'Group-1', null ); insert into t1 values ( 3, 'Group-2', 'a' ); insert into t1 values ( 4, 'Group-2', 'a' ); insert into t1 values ( 5, 'Group-2', 'z' ); insert into t1 values ( 6, 'Group-2', 'z' ); insert into t1 values ( 7, 'Group-2', null ); insert into t1 values ( 8, 'Group-3', 'A' ); insert into t1 values ( 9, 'Group-3', 'A' ); insert into t1 values (10, 'Group-3', 'Z' ); commit; set null '(null)'