--- 1

select id, imie, nazwisko, coalesce(sum(saldo),0)
from Klient left join Konto on id = idklienta
group by id, imie, nazwisko;

--- 2

select nadawca 
from Przelew
group by nadawca
having count(distinct odbiorca) < 6;

[dla pedantów:

union (select numer from Konto
       where numer not in (select nadawca from Przelew))
]

--- 3

select numer
from Konto
where numer in (select nadawca from Przelew)
   or numer in (select odbiorca from Przelew);

select nadawca from przelew  --??? distinct
intersect
select odbiorca from przelew;

--- 4

select nadawca
from Przelew
group by nadawca
having count(distinct odbiorca) = (select count(*) - 1 from Konto);

--- 5

create view czworki as
select * from przelewy p1
where (nadawca,odbiorca,data) in
         (select nadawca,odbiorca,data
          from transakcja
          where odbiorca = p1.odbiorca
          order by kwota desc limit 4);

select odbiorca
from czworki 
group by odbiorca
having sum(kwota) > 10000;