Firebird 3.0

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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

WITH  
            CT      AS (SELECT ID_KURSZEILE, datum, fk_kontrakt,
                            ROW_NUMBER() OVER(PARTITION BY fk_kontrakt ORDER BY datum) AS FirstDate,
                            ROW_NUMBER() OVER(PARTITION BY fk_kontrakt ORDER BY datum DESC) AS LastDate
                            FROM TBKURSZEILEN),
            MX      AS (SELECT MAX(datum) AS MaxStart FROM CT WHERE FirstDate=1),
            MI      AS (SELECT MIN(datum) AS MinLast FROM CT WHERE LastDate=1),
            MU      AS (SELECT MaxStart, MinLast FROM MX INNER JOIN MI ON 1=1)
     
    SELECT T.ID_KURSZEILE, T.datum, T.fk_kontrakt
    FROM TBKURSZEILEN AS T
    INNER JOIN CT ON CT.ID_KURSZEILE=T.ID_KURSZEILE
    INNER JOIN MU ON T.datum BETWEEN MU.MaxStart AND MU.MinLast
    WHERE T.fk_kontrakt IN (5,6)
    ORDER BY T.datum


ID_KURSZEILE
DATUM
FK_KONTRAKT


5
2024-05-01
5

11
2024-05-01
6

9
2024-05-15
5

6
2024-06-01
5

12
2024-06-01
6

10
2024-06-15
5

7
2024-07-01
5

13
2024-07-01
6

8
2024-08-01
5

14
2024-08-01
6
ID_KURSZEILE	DATUM	FK_KONTRAKT
5	2024-05-01	5
11	2024-05-01	6
9	2024-05-15	5
6	2024-06-01	5
12	2024-06-01	6
10	2024-06-15	5
7	2024-07-01	5
13	2024-07-01	6
8	2024-08-01	5
14	2024-08-01	6