SQL Server 2017

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create table tablename (customerid int,  channel varchar(10));
insert into tablename values 
(1, 'online'), (1, 'store'), (2, 'store'),  (3, 'online'), (3, 'online');
5 rows affected

select distinct customerid 
from tablename t
where not exists (
  select 1 from tablename where customerid = t.customerid and channel = 'store'
)


customerid


3


select distinct customerid 
from tablename t
where 
  (select count(distinct channel) from tablename where customerid = t.customerid) = 2


customerid


1


select distinct customerid, 'online' channel
from tablename t
where not exists (
  select 1 from tablename where customerid = t.customerid and channel = 'store'
)
union all
select distinct customerid, 'both' channel
from tablename t
where 
  (select count(distinct channel) from tablename where customerid = t.customerid) = 2


customerid
channel


3
online

1
both