Select Case query not working

New to SQL and in the process of developing query, with the to replicate part of an ETL process.

A billing code1 = bf, is then set to debt code 1. If billing field (debt code) exceeds 100 characters then add '*' as prefix.

However query will fall down because billing code1 = bf returns a single resultset to a debt code1 that returns large resultset.

select 
    case when len(format) > 100 then left(format, 100) + '*' 
    else forma开发者_运维知识库t end as format
from (select case when exists (select _hs_eb_code1 from hbl_cat where hs_eb_code = 'bf)
                  then tbm_bllgrp._hs_eb_det1 end) as format
from tbm_bllgrp

Ideas welcomed.

---

select 
case when len(format) > 100 then left(format, 100) + '*' 
else format end as format
from tbm_bllgrp b
    inner join hbl_cat hc on hc._hs_eb_code1 = b._hs_eb_det1

This is just a guess since I don't have your exactl schema and expected output, but I hope this gives you some ideas