sql join and conditional update

Hi list,
  Apologies if this is the wrong list to post to, but i have seen a few sql Qs 
here.
   
  could some one help me with an sql problem i have been toiling with for a 
while now.
  I am trying to update a field (dslqts) in a file by subtracting the value of 
"qty" from another file.  However, if the result is a negative number i want 
the field to be set to zero.
   
  My code...
   
  update "CRT.LIB"/dudstock 
  set dslqts=dslqts- 
  case when dslqts-(select coalesce(qty,0) from "CRT.LIB"/crt 
      where code=dsprod)>=0 
    then (select coalesce(qty,0) from "CRT.LIB"/crt 
      where code=dsprod) 
    else dslqts 
  end 
   
  I expected the case/when/then section to perform the sum, and take care of 
any unmatched records (by subtracting zero), then the else statement will deal 
with sums resulting in a negitave and making the result = zero.  But when run 
it changes every unmatched record to zero.
   
  Any help very appriciated...
  Thanks,
  Colin.

                
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