Oracle 21c

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ALTER SESSION SET NLS_DATE_FORMAT = 'YYYY-MM-DD HH24:MI:SS';
CREATE TABLE table_name (dt, STORE, PRODUCT) AS
SELECT DATE '2020-01-01', 'Store1', 'Product1' FROM DUAL UNION ALL
SELECT DATE '2020-01-01', 'Store1', 'Product2' FROM DUAL UNION ALL
SELECT DATE '2020-01-01', 'Store1', 'Product3' FROM DUAL UNION ALL
SELECT DATE '2020-01-01', 'Store1', 'Product4' FROM DUAL UNION ALL
SELECT DATE '2020-02-01', 'Store1', 'Product5' FROM DUAL UNION ALL
SELECT DATE '2020-02-01', 'Store1', 'Product6' FROM DUAL UNION ALL
SELECT DATE '2020-02-01', 'Store1', 'Product7' FROM DUAL UNION ALL
SELECT DATE '2020-02-01', 'Store1', 'Product8' FROM DUAL UNION ALL
SELECT DATE '2020-01-01', 'Store2', 'Product1' FROM DUAL UNION ALL
SELECT DATE '2020-01-01', 'Store2', 'Product2' FROM DUAL UNION ALL
SELECT DATE '2020-01-01', 'Store2', 'Product3' FROM DUAL UNION ALL
SELECT DATE '2020-01-01', 'Store2', 'Product4' FROM DUAL UNION ALL
SELECT DATE '2020-01-03', 'Store2', 'Product1' FROM DUAL UNION ALL
SELECT DATE '2020-03-01', 'Store2', 'Product2' FROM DUAL UNION ALL
SELECT DATE '2020-03-02', 'Store2', 'Product8' FROM DUAL UNION ALL
SELECT DATE '2020-03-03', 'Store2', 'Product10' FROM DUAL UNION ALL
SELECT DATE '2020-08-06', 'Store2', 'Product6' FROM DUAL UNION ALL
SELECT DATE '2020-11-08', 'Store2', 'Product7' FROM DUAL;
18 rows affected

SELECT dt,
       store,
       product,
       DENSE_RANK() OVER (ORDER BY store) || LPAD('e', mn - 1, 'e') AS tt
FROM   table_name
       MATCH_RECOGNIZE(
         PARTITION BY store
         ORDER BY dt, product
         MEASURES
           MATCH_NUMBER() AS mn
         ALL ROWS PER MATCH
         PATTERN (first_row within_20_day* )
         DEFINE
           within_20_day AS dt <= PREV(dt) + INTERVAL '20' DAY
       )


DT
STORE
PRODUCT
TT


2020-01-01 00:00:00
Store1
Product1
1

2020-01-01 00:00:00
Store1
Product2
1

2020-01-01 00:00:00
Store1
Product3
1

2020-01-01 00:00:00
Store1
Product4
1

2020-02-01 00:00:00
Store1
Product5
1e

2020-02-01 00:00:00
Store1
Product6
1e

2020-02-01 00:00:00
Store1
Product7
1e

2020-02-01 00:00:00
Store1
Product8
1e

2020-01-01 00:00:00
Store2
Product1
2

2020-01-01 00:00:00
Store2
Product2
2

2020-01-01 00:00:00
Store2
Product3
2

2020-01-01 00:00:00
Store2
Product4
2

2020-01-03 00:00:00
Store2
Product1
2

2020-03-01 00:00:00
Store2
Product2
2e

2020-03-02 00:00:00
Store2
Product8
2e

2020-03-03 00:00:00
Store2
Product10
2e

2020-08-06 00:00:00
Store2
Product6
2ee

2020-11-08 00:00:00
Store2
Product7
2eee


SELECT dt,
       store,
       product,
       DENSE_RANK() OVER (ORDER BY store)
       || LPAD(
            'e',
            SUM(has_changed_grp) OVER (PARTITION BY store ORDER BY dt, product) - 1,
            'e'
          ) AS tt
FROM   (
  SELECT dt,
         store,
         product,
         CASE
         WHEN dt <= LAG(dt) OVER (PARTITION BY store ORDER BY dt, product)
                    + INTERVAL '20' DAY
         THEN 0
         ELSE 1
         END AS has_changed_grp
  FROM   table_name
)


DT
STORE
PRODUCT
TT


2020-01-01 00:00:00
Store1
Product1
1

2020-01-01 00:00:00
Store1
Product2
1

2020-01-01 00:00:00
Store1
Product3
1

2020-01-01 00:00:00
Store1
Product4
1

2020-02-01 00:00:00
Store1
Product5
1e

2020-02-01 00:00:00
Store1
Product6
1e

2020-02-01 00:00:00
Store1
Product7
1e

2020-02-01 00:00:00
Store1
Product8
1e

2020-01-01 00:00:00
Store2
Product1
2

2020-01-01 00:00:00
Store2
Product2
2

2020-01-01 00:00:00
Store2
Product3
2

2020-01-01 00:00:00
Store2
Product4
2

2020-01-03 00:00:00
Store2
Product1
2

2020-03-01 00:00:00
Store2
Product2
2e

2020-03-02 00:00:00
Store2
Product8
2e

2020-03-03 00:00:00
Store2
Product10
2e

2020-08-06 00:00:00
Store2
Product6
2ee

2020-11-08 00:00:00
Store2
Product7
2eee

DT	STORE	PRODUCT	TT
2020-01-01 00:00:00	Store1	Product1	1
2020-01-01 00:00:00	Store1	Product2	1
2020-01-01 00:00:00	Store1	Product3	1
2020-01-01 00:00:00	Store1	Product4	1
2020-02-01 00:00:00	Store1	Product5	1e
2020-02-01 00:00:00	Store1	Product6	1e
2020-02-01 00:00:00	Store1	Product7	1e
2020-02-01 00:00:00	Store1	Product8	1e
2020-01-01 00:00:00	Store2	Product1	2
2020-01-01 00:00:00	Store2	Product2	2
2020-01-01 00:00:00	Store2	Product3	2
2020-01-01 00:00:00	Store2	Product4	2
2020-01-03 00:00:00	Store2	Product1	2
2020-03-01 00:00:00	Store2	Product2	2e
2020-03-02 00:00:00	Store2	Product8	2e
2020-03-03 00:00:00	Store2	Product10	2e
2020-08-06 00:00:00	Store2	Product6	2ee
2020-11-08 00:00:00	Store2	Product7	2eee