declare @undoc int = 1194504
declare @key varchar(100) = ''
declare @diff numeric(16,4) = 0
declare @rowdiff numeric(16,4) = 0

--drop table #tmpDms

select * into #tmpDms from DMS where UNDOC = @undoc order by KMAT,DB

update dms
set 
@diff = CASE WHEN (dms.KMAT+dms.db) != @key THEN chet.diff ELSE @diff - @rowdiff END,
@key = dms.KMAT+dms.db,
@rowdiff = CASE WHEN @diff > dms.kol_p THEN dms.kol_p ELSE @diff END,
kol = kol_p - @rowdiff
from (select chet.kmat, chet.db, sum(dms.dms_kol) - sum(isnull(chet.chet_kol,0)) as diff -- Было - стало, уменьшение будет > 0
from (select A.kmat, A.db, sum(isnull(A.kol,0)) as chet_kol
from chet A
where pr1 = @undoc 
group by A.kmat, A.db) as chet 
left join (select #tmpDms.kmat, #tmpDms.db, sum(#tmpDms.kol_p) as dms_kol
from #tmpDms 
group by #tmpDms.kmat, #tmpDms.db) as dms 
on dms.kmat = chet.kmat  and dms.db = chet.db
group by chet.kmat, chet.db) chet 
join #tmpDms dms on chet.kmat = dms.kmat and chet.db = dms.db

update DMS
set KOL = #tmpDms.KOL
from DMS join #tmpDms on DMS.UNDOC = #tmpDms.UNDOC and DMS.NPP = #tmpDms.NPP

drop table #tmpDms