merging rows into 1 column - sql

Question

I have a table with 2 columns
Input
Col 1 ---- Col 2
1 ---- aaaa
1 ---- bbbb
1 ---- cccc
2 ---- dddd
2 ---- eeee
2 ---- ffff
2 ---- gggg
Output
Col 1 ---- Col 2
1 ---- aaaabbbbcccc
2 ---- ddddeeeeffffgggg
I was thinking of doing several self joins, but doesnt seem efficient. Any ideas on how the sql has to be written?

Answer 1

Ok, I'll bite. Instead of stragg, try listagg (in 11.2):
create table tst1
(
pid number,
val varchar2(10)
);
insert into tst1 values(1, 'Rec1');
insert into tst1 values(1, 'Rec2');
insert into tst1 values(1, 'Rec3');
insert into tst1 values(2, 'Rec1');
insert into tst1 values(2, 'Rec2');
commit;
select pid, listagg(val, ':') within group(order by val) as "The List"
from tst1
group by pid;
And you get:
pid The List
1 Rec1:Rec2:Rec3
2 Rec1:Rec2
If you change the order by to "order by val desc" you'd get
pid The List
1 Rec3:Rec2:Rec1
2 Rec2:Rec1

Answer 2

This is a version that will work in Oracle 9i and up.
create table foo (
key_column number,
val_column varchar2(4)
);
insert into foo values (1, 'aaaa');
insert into foo values (1, 'bbbb');
insert into foo values (1, 'cccc');
insert into foo values (2, 'dddd');
insert into foo values (2, 'eeee');
insert into foo values (2, 'ffff');
insert into foo values (2, 'gggg');
select key_column
, replace(max(sys_connect_by_path(val_column, ',')), ',') combined
from (select key_column
, val_column
, row_number() over (partition by key_column order by val_column) cur
, row_number() over (partition by key_column order by val_column) - 1 prev
from foo) foo
group by key_column
connect by prior cur = prev and prior key_column = key_column
start with cur = 1;
key_column | val_column
--------------------------
1 | aaaabbbbcccc
2 | ddddeeeeffffgggg