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declare @Yourtable table (part_no varchar(100), store varchar(100));
insert into @Yourtable values ('123','store a'),('123','store b'),('456','store b'),('456','store c'),('999','store c');
select part_no, string_agg(store,',') as stores
from @Yourtable t
group by part_no;
| part_no | stores |
|---|---|
| 123 | store a,store b |
| 456 | store b,store c |
| 999 | store c |