You could try,

select *

into D

from A

where not exists (select 'x' from C where C.key=A.key)

union

select *

into D

from B

where not exists (select 'x' from C where C.key=B.key)

I Also had this problem of not seeing (missing) any MS SQL counters in the perfmon tool,

I followed many recommendations and at last this one

and that did the trick.

Thanks...