This query gives an error:

select ep, 
  case
    when ob is null and b2b_ob is null then 'a'
    when ob is not null or b2b_ob is not null then 'b'
    else null
  end as type,
  sum(b2b_d + b2b_t - b2b_i) as sales
from table
where ...
group by ep, type

Error: ORA-00904: "TYPE": invalid identifier

When I run it with group by ep, the error message becomes:

ORA-00979: not a GROUP BY expression

The whole query works OK if I remove the lines sum(b2b_d+b2b_t-b2b_i) as sales and group by ..., so the problem should be related to SUM and GROUP BY functions. How can I make this work? Thanks in advance for your help.