Oracle 18c

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ALTER SESSION SET NLS_DATE_FORMAT = 'YYYY-MM-DD HH24:MI:SS';
CREATE TABLE table_name (Groups, Sort_Sequence, Stop_number) AS
SELECT 'A', 1, 20000 FROM DUAL UNION ALL
SELECT 'A', 2, 20231 FROM DUAL UNION ALL
SELECT 'A', 3, 20001 FROM DUAL UNION ALL
SELECT 'A', 4, 21567 FROM DUAL UNION ALL
SELECT 'A', 5, 20002 FROM DUAL UNION ALL
SELECT 'B', 1, 20000 FROM DUAL UNION ALL
SELECT 'B', 2, 20001 FROM DUAL UNION ALL
SELECT 'B', 3, 20231 FROM DUAL UNION ALL
SELECT 'B', 4, 20002 FROM DUAL UNION ALL
SELECT 'B', 5, 20003 FROM DUAL UNION ALL
SELECT 'B', 6, 20004 FROM DUAL UNION ALL
SELECT 'B', 7, 21567 FROM DUAL UNION ALL
SELECT 'B', 8, 20005 FROM DUAL UNION ALL
SELECT 'B', 9, 20006 FROM DUAL;
14 rows affected

SELECT Groups,
       Sort_Sequence,
       Stop_number
FROM   (
  SELECT Groups,
         Sort_Sequence,
         Stop_number,
         MIN(CASE stop_number WHEN 20231 THEN sort_sequence END)
           OVER (PARTITION BY groups) AS start_sequence,
         MAX(CASE stop_number WHEN 21567 THEN sort_sequence END)
           OVER (PARTITION BY groups) AS end_sequence
  FROM   table_name
)
WHERE  sort_sequence BETWEEN start_sequence AND end_sequence
ORDER BY
       groups,
       sort_sequence


GROUPS
SORT_SEQUENCE
STOP_NUMBER


A
2
20231

A
3
20001

A
4
21567

B
3
20231

B
4
20002

B
5
20003

B
6
20004

B
7
21567


SELECT Groups,
       Sort_Sequence,
       Stop_number
FROM   (
  SELECT Groups,
         Sort_Sequence,
         Stop_number,
         SUM(CASE stop_number WHEN 20231 THEN 1 WHEN 21567 THEN -1 ELSE 0 END)
           OVER (PARTITION BY groups ORDER BY sort_sequence) AS num_start_stop
  FROM   table_name
)
WHERE  num_start_stop > 0
OR     (num_start_stop = 0 AND stop_number = 21567)


GROUPS
SORT_SEQUENCE
STOP_NUMBER


A
2
20231

A
3
20001

A
4
21567

B
3
20231

B
4
20002

B
5
20003

B
6
20004

B
7
21567