postgres=# create table kunden(id int primary key, name text);
CREATE TABLE
postgres=# create table auftraege (aid int primary key, kid int references kunden, menge int);
CREATE TABLE
postgres=# insert into kunden values (1, 'kunde1');
INSERT 0 1
postgres=# insert into kunden values (2, 'kunde2');
INSERT 0 1
postgres=# insert into auftraege values (1,1,5);
INSERT 0 1
postgres=# insert into auftraege values (2,2,8);
INSERT 0 1
postgres=# insert into auftraege values (3,2,8);
INSERT 0 1
postgres=# select k.id, k.name, sum(menge) from kunden k left join auftraege a on k.id=a.kid group by k.id, k.name having sum(menge) > 10;
id | name | sum
----+--------+-----
2 | kunde2 | 16
(1 row)