Db2 11.1

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create table t (
  customer_id int,
  offer_1 varchar(10),
  offer_2 varchar(10),
  offer_3 varchar(10)
)
insert into t (customer_id, offer_1, offer_2, offer_3)
values (111, 'A01', '001', 'B01')
insert into t (customer_id, offer_1, offer_2, offer_3)
values (222, 'A01', 'B01', '001')
insert into t (customer_id, offer_1, offer_2, offer_3)
values (333, 'A02', '001', 'B01')
select * from t


CUSTOMER_ID
OFFER_1
OFFER_2
OFFER_3


111
A01
001
B01

222
A01
B01
001

333
A02
001
B01


select count(distinct offer_combo) as distinct_offers
from (
  select listagg(offer, '/') within group (order by offer) as offer_combo
  from (
    select customer_id, offer_1 as offer from t
    union all select customer_id, offer_2 from t
    union all select customer_id, offer_3 from t
  ) x
  group by customer_id
) y


DISTINCT_OFFERS


2