SQL Update using Case results

Question

I need to update a value in file1 with the contents of a field in another file2 with a matching key, but only if a row is found in file2 that matches. Otherwise, update file1 field with a 'Q' literal. This works, but seems redundant, and takes too long? Suggestions?

update ZXU                                                 
set XUATTN =  case when (select count(*) from ZXK       
                         where XKUSER = 'TOMTEST') > 0 
                  then (select XKAUTH from ZXK         
                         where XKUSER = 'TOMTEST')     
                  else 'Q'                             
             end                                       
where XUUSER='TOMTEST'                                     

Answer 1

You can use COALESCE() :

update ZXU
    set XUATTN = COALESCE( (select k.XKAUTH from ZXK k where k.XKUSER = ZHU.XUUSER), 'Q')
    where XUUSER = 'TOMTEST';

I will note that this (and your code) will generate an error if the subquery returns more than one row.