Firebird 3.0

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select rdb$get_context('SYSTEM', 'ENGINE_VERSION') as version
     , rdb$character_set_name
from rdb$database;


VERSION
RDB$CHARACTER_SET_NAME


3.0.10
UTF8


create table seller (
  seller_id integer constraint pk_seller primary key,
  seller varchar(100) not null
)
create table docs (
  doc_id integer constraint pk_docs primary key,
  doc_summa integer not null,
  seller_id integer constraint fk_docs_seller references seller (seller_id)
)
create table paym (
  paid_id integer constraint pk_paym primary key,
  doc_id integer constraint fk_paym_docs references docs (doc_id),
  paid integer not null
)
execute block
as
begin
  insert into seller (seller_id, seller) values (8, 'Firm1');
  insert into seller (seller_id, seller) values (45, 'Firm2');

  insert into docs (doc_id, doc_summa, seller_id) values (346, 1000, 8);
  insert into docs (doc_id, doc_summa, seller_id) values (347, 600, 45);
  insert into docs (doc_id, doc_summa, seller_id) values (348, 800, 45);

  insert into paym (paid_id, doc_id, paid) values (1, 346, 100);
  insert into paym (paid_id, doc_id, paid) values (2, 346, 100);
  insert into paym (paid_id, doc_id, paid) values (3, 347, 200);
  insert into paym (paid_id, doc_id, paid) values (4, 348, 100);
  insert into paym (paid_id, doc_id, paid) values (5, 348, 50);
end
10 rows affected

-- original query
SELECT
 s.seller_id, 
 sum(d.doc_summa) as summa,
 sum(p.paid) as paid,
 sum(d.doc_summa)-sum(p.paid) as debt

FROM seller s Left Join Docs d  on s.seller_id=d.seller_id
             Left Join  paym p  on p.doc_id= d.doc_id

GROUP BY s.seller_id


SELLER_ID
SUMMA
PAID
DEBT


8
2000
200
1800

45
2200
350
1850


with consolidated_paym as (
  select 
    doc_id, 
    sum(paid) as total_paid
  from paym
  group by doc_id
)
select
  s.seller_id,
  sum(d.doc_summa) as summa,
  sum(p.total_paid) as paid,
  sum(d.doc_summa - p.total_paid) as debt
from seller s
left join docs d
  on d.seller_id = s.seller_id
left join consolidated_paym as p
  on p.doc_id = d.doc_id
group by s.seller_id


SELLER_ID
SUMMA
PAID
DEBT


8
1000
200
800

45
1400
350
1050