how to collect values from multiple rows into a single, comma delimited string. (using stragg = string aggregation)

Prerequisites Of STRAGG (Tom Kyte's)
==========================================

create or replace type stragg_type as object
(
string varchar2(4000),

static function ODCIAggregateInitialize
( sctx in out stragg_type )
return number ,

member function ODCIAggregateIterate
( self in out stragg_type ,
value in varchar2
) return number ,

member function ODCIAggregateTerminate
( self in stragg_type,
returnvalue out varchar2,
flags in number
) return number ,

member function ODCIAggregateMerge
( self in out stragg_type,
ctx2 in stragg_type
) return number
);
/

create or replace type body stragg_type
is

static function ODCIAggregateInitialize
( sctx in out stragg_type )
return number
is
begin

sctx := stragg_type( null ) ;

return ODCIConst.Success ;

end;

member function ODCIAggregateIterate
( self in out stragg_type ,
value in varchar2
) return number
is
begin

self.string := self.string || ',' || value ;

return ODCIConst.Success;

end;

member function ODCIAggregateTerminate
( self in stragg_type ,
returnvalue out varchar2 ,
flags in number
) return number
is
begin

returnValue := ltrim( self.string, ',' );

return ODCIConst.Success;

end;

member function ODCIAggregateMerge
( self in out stragg_type ,
ctx2 in stragg_type
) return number
is
begin

self.string := self.string || ctx2.string;

return ODCIConst.Success;

end;

end;
/

create or replace function stragg
( input varchar2 )
return varchar2
deterministic
parallel_enable
aggregate using stragg_type
;
/

Data:-
=========
GROUP_KEY VAL
---------- ----------
Group 1 a

Group 2 a
Group 2 b

Group 3 a
Group 3 b

use of stragg
================
select
group_key ,
STRAGG( VAL ) as string
from
t
group by
group_key
;


select
group_key ,
stragg( DISTINCT val ) as string
from
t
group by
group_key
;