Firebird 3.0

By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.
CREATE TABLE TBKURSZEILEN(
  ID_KURSZEILE integer generated by default as identity primary key,
  datum date,
  fk_kontrakt int
);
EXECUTE BLOCK AS BEGIN
INSERT INTO TBKURSZEILEN(datum, fk_kontrakt) VALUES('2024-01-01', 5);
INSERT INTO TBKURSZEILEN(datum, fk_kontrakt) VALUES('2024-02-01', 5);
INSERT INTO TBKURSZEILEN(datum, fk_kontrakt) VALUES('2024-03-01', 5);
INSERT INTO TBKURSZEILEN(datum, fk_kontrakt) VALUES('2024-04-01', 5);
INSERT INTO TBKURSZEILEN(datum, fk_kontrakt) VALUES('2024-05-01', 5);
INSERT INTO TBKURSZEILEN(datum, fk_kontrakt) VALUES('2024-06-01', 5);
INSERT INTO TBKURSZEILEN(datum, fk_kontrakt) VALUES('2024-07-01', 5);
INSERT INTO TBKURSZEILEN(datum, fk_kontrakt) VALUES('2024-08-01', 5);
INSERT INTO TBKURSZEILEN(datum, fk_kontrakt) VALUES('2024-05-15', 5);
INSERT INTO TBKURSZEILEN(datum, fk_kontrakt) VALUES('2024-06-15', 5);
INSERT INTO TBKURSZEILEN(datum, fk_kontrakt) VALUES('2024-05-01', 6);
INSERT INTO TBKURSZEILEN(datum, fk_kontrakt) VALUES('2024-06-01', 6);
INSERT INTO TBKURSZEILEN(datum, fk_kontrakt) VALUES('2024-07-01', 6);
INSERT INTO TBKURSZEILEN(datum, fk_kontrakt) VALUES('2024-08-01', 6);
INSERT INTO TBKURSZEILEN(datum, fk_kontrakt) VALUES('2024-09-01', 6);
INSERT INTO TBKURSZEILEN(datum, fk_kontrakt) VALUES('2024-10-01', 6);
INSERT INTO TBKURSZEILEN(datum, fk_kontrakt) VALUES('2024-11-01', 6);
INSERT INTO TBKURSZEILEN(datum, fk_kontrakt) VALUES('2024-12-01', 6);
INSERT INTO TBKURSZEILEN(datum, fk_kontrakt) VALUES('2024-08-15', 6);
INSERT INTO TBKURSZEILEN(datum, fk_kontrakt) VALUES('2024-09-15', 6);
END
20 rows affected

select datum
from TBKURSZEILEN
where fk_kontrakt in (5,6)
group by 1
having count(*) = 2


DATUM


2024-05-01

2024-06-01

2024-07-01

2024-08-01