Resolved: SQL query to count number of accounts based on criteria A only, B only and both A&B

--if CTE not supported, just change to a subquery
with comm_type as
(
    select distinct t1.ACCOUNT_ID
        , case
            when t1.TYPE_OF_COMM like 'Email' and t2.TYPE_OF_COMM is null then 'Email Only'
            when t1.TYPE_OF_COMM like 'Letter' and t2.TYPE_OF_COMM is null then 'Letter Only'
            when t1.TYPE_OF_COMM is not null and t2.TYPE_OF_COMM is not null then 'Both Letters and Mails'
            else 'Unkown'
          end TYPE_OF_COMMS
    from tbl t1
    left join tbl t2
        on t1.ACCOUNT_ID = t2.ACCOUNT_ID
        and t1.TYPE_OF_COMM <> t2.TYPE_OF_COMM
)
select TYPE_OF_COMMS 
    , count(distinct ACCOUNT_ID)
from comm_type
group by TYPE_OF_COMMS