How to retreive deleted records in a trigger on a table with composite primary key
If I'm reading that right, this should work:

select
  from deleted d
  where not exists (select 1 from inserted i where i.a=d.a and i.b=d.b)




Regarding srutzky's comment 


  "And for an UPDATE, both inserted and deleted tables have the exact same rows in them (as you cannot update a row that did not exist)."


This is not true in the case of updatable primary keys.

create table dbo.srutzky (a int not null
  , b nvarchar(44) not null
  , c bit not null
  , constraint pk_srutzky primary key clustered (a,b)
)

insert into dbo.srutzky (a,b,c) values
 (1,'for an UPDATE',1)
,(2,'both inserted and deleted',1)
,(3,'have the exact same rows in them ',1)

update dbo.srutzky 
  set a=4
    , b = 'might not have the exact same rows in them' 
  output deleted.a as deleted_a
       , deleted.b as deleted_b
       , inserted.a as inserted_a
       , inserted.b as inserted_b
  where a = 3


Results 


deleted_a   deleted_b                                    inserted_a  inserted_b
----------- -------------------------------------------- ----------- --------------------------------------------
3           have the exact same rows in them             4           might not have the exact same rows in them