Db2 11.1

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CREATE TABLE table_name
    (Customer_ID int, Offer_1 varchar(3), Offer_2 varchar(3), Offer_3 varchar(3))
;
    

INSERT INTO table_name
    (Customer_ID, Offer_1, Offer_2, Offer_3)
VALUES
    (111, 'A01', '001', 'B01'),
    (222, 'A01', 'B01', '001'),
    (333, 'A02', '001', 'B01')
;
with u as
(select Customer_ID, Offer_1 as Offer from table_name union all
 select Customer_ID, Offer_2 as Offer from table_name union all
 select Customer_ID, Offer_3 as Offer from table_name)
select distinct listagg(Offer, ',') within group(order by Offer) from u
group by Customer_ID


1


001,A01,B01

001,A02,B01