Oracle 18c

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ALTER SESSION SET NLS_DATE_FORMAT = 'YYYY-MM-DD HH24:MI:SS';
CREATE TABLE table_name (trans_id, customer_id, trans_date) AS
  SELECT '001', '1101',  DATE '2020-11-02' FROM DUAL UNION ALL
  SELECT '002', '1101',  DATE '2020-11-14' FROM DUAL UNION ALL
  SELECT '003', '1101',  DATE '2020-11-18' FROM DUAL UNION ALL
  SELECT '004', '1101',  DATE '2021-12-04' FROM DUAL UNION ALL
  SELECT '005', '1101',  DATE '2021-12-05' FROM DUAL UNION ALL
  SELECT '006', '1101',  DATE '2021-12-08' FROM DUAL UNION ALL
  SELECT '007', '1101',  DATE '2021-01-17' FROM DUAL UNION ALL
  SELECT '008', '1101',  DATE '2021-05-01' FROM DUAL UNION ALL
  SELECT '009', '1101',  DATE '2021-05-04' FROM DUAL UNION ALL
  SELECT '010', '1102',  DATE '2021-03-02' FROM DUAL UNION ALL
  SELECT '011', '1102',  DATE '2021-03-08' FROM DUAL UNION ALL
  SELECT '012', '1102',  DATE '2021-04-01' FROM DUAL UNION ALL
  SELECT '013', '1102',  DATE '2021-04-02' FROM DUAL UNION ALL
  SELECT '014', '1102',  DATE '2021-04-12' FROM DUAL UNION ALL
  SELECT '015', '1102',  DATE '2021-04-29' FROM DUAL UNION ALL
  SELECT '016', '1102',  DATE '2021-06-10' FROM DUAL UNION ALL
  SELECT '017', '1102',  DATE '2021-06-12' FROM DUAL;
17 rows affected

SELECT *
FROM   table_name
MATCH_RECOGNIZE(
  PARTITION BY customer_id
  ORDER BY trans_date
  MEASURES
    MATCH_NUMBER() AS grp
  ALL ROWS PER MATCH
  PATTERN ( within_30_days+ )
  DEFINE
    within_30_days AS trans_date <= FIRST(trans_date) + INTERVAL '30' DAY
)


CUSTOMER_ID
TRANS_DATE
GRP
TRANS_ID


1101
2020-11-02 00:00:00
1
001

1101
2020-11-14 00:00:00
1
002

1101
2020-11-18 00:00:00
1
003

1101
2021-01-17 00:00:00
2
007

1101
2021-05-01 00:00:00
3
008

1101
2021-05-04 00:00:00
3
009

1101
2021-12-04 00:00:00
4
004

1101
2021-12-05 00:00:00
4
005

1101
2021-12-08 00:00:00
4
006

1102
2021-03-02 00:00:00
1
010

1102
2021-03-08 00:00:00
1
011

1102
2021-04-01 00:00:00
1
012

1102
2021-04-02 00:00:00
2
013

1102
2021-04-12 00:00:00
2
014

1102
2021-04-29 00:00:00
2
015

1102
2021-06-10 00:00:00
3
016

1102
2021-06-12 00:00:00
3
017

CUSTOMER_ID	TRANS_DATE	GRP	TRANS_ID
1101	2020-11-02 00:00:00	1	001
1101	2020-11-14 00:00:00	1	002
1101	2020-11-18 00:00:00	1	003
1101	2021-01-17 00:00:00	2	007
1101	2021-05-01 00:00:00	3	008
1101	2021-05-04 00:00:00	3	009
1101	2021-12-04 00:00:00	4	004
1101	2021-12-05 00:00:00	4	005
1101	2021-12-08 00:00:00	4	006
1102	2021-03-02 00:00:00	1	010
1102	2021-03-08 00:00:00	1	011
1102	2021-04-01 00:00:00	1	012
1102	2021-04-02 00:00:00	2	013
1102	2021-04-12 00:00:00	2	014
1102	2021-04-29 00:00:00	2	015
1102	2021-06-10 00:00:00	3	016
1102	2021-06-12 00:00:00	3	017