Postgres 11

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create table the_table (Inv_ID varchar(20), Item_Code varchar(20),  Item_Q int);

CREATE TABLE

insert into the_table
values
('A-800', '101010', 24),
('A-801', '101010', 24),
('A-802', '202020', 9),
('A-803', '101010', 18),
('A-804', '202020', 9),
('A-805', '202020', 9),
('A-806', '101010', 18);

INSERT 0 7

select string_agg(inv_id, ',' order by inv_id) as inv_ids
from the_table
group by item_code, item_q
order by inv_ids;

inv_ids

A-800,A-801

A-802,A-804,A-805

A-803,A-806

SELECT 3