postgresql - SQL Multiple update record from SELECT WHERE -

i need help.

i want totally quantity of material same id in bill.

like this

bill no. 001 jellopy x1ea jellopy x1ea jellopy x1ea zargon x1ea 

result should be

bill no. 001 jellopy x3ea zargon x1ea 

so,i want this

1.select+sum 2.update 3.delete duplicate

here old (dbill) table

dbill

id  mat  qty 01  a1   1 01  a1   1 01  a1   1 01  a2   1  id  mat  qty [01  a1   1] < same mat&id = sum [01  a1   1] < same mat&id = sum [01  a1   1] < same mat&id = sum  01  a2   1 

and sqlfiddle test http://sqlfiddle.com/#!15/b30a3c

1 sum quantity same material

select mat,id,sum(qty) result dbill group id,mat 

[2] got result this

id  mat  qty 01  a1   3 01  a2   1 

[3] so... update this

update dbill set qty = result dbill t inner join (select mat,id,sum(qty) result dbill group id,mat) s on s.id = t.id , s.mat = t.mat 

the problem when [3] update here result

id  mat  qty 01  a1   3 01  a1   3 01  a1   3 01  a2   3 

because correct result should be

id  mat  qty 01  a1   3 01  a1   3 01  a1   3 01  a2   1     < 

thank understand , answer ...

for me query seems ok replace update dbill update t

update t <---- set qty = result dbill t inner join (select mat,id,sum(qty) result  dbill  group id,mat) s on s.id = t.id , s.mat = t.mat