create or replace type stringlist_o as object( text varchar2(4000) )  

create or replace type stringlist_t as table of stringlist_o 

create or replace type stringlist_list_t as table of stringlist_t 

create table t1 (val varchar2(4000), room_uid integer ) 

insert into t1 (val, room_uid ) values ('test text', 621 ) 

commit 

with X as 
( select stringlist_t(stringlist_o(val)) t1_val, room_uid from t1) 
select room_uid, 
       cast( collect(t1_val) as stringlist_list_t) t1_val_collection 
 from X 
 group by room_uid

 select room_uid, cast(collect(t1_val) as stringlist_list_t) from ( 
    select room_uid, stringlist_t( stringlist_o( val ) ) t1_val from t1 
    ) group by room_uid