MySQL 8.0

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CREATE TABLE datetable (
  `Id` INTEGER,
  `unique_identifier` INTEGER,
  `reference_date` Date
);

INSERT INTO datetable
  (`Id`, `unique_identifier`, `reference_date`)
VALUES
  ('1', '123456789', '2020-01-05'),
  ('2', '123456789', '2020-02-20'),
  ('3', '123456789', '2020-06-29'),
  ('4', '987654321', '2020-02-21'),
  ('5', '987654321', '2020-03-30');
SELECT
`Id`, `reference_date`
,IF(@unique_identifier = `unique_identifier`,IF(DATEDIFF(`reference_date`,@date) > 60,1,0),1) 60_days_since_seen
,@unique_identifier := `unique_identifier` as 'unique_identifier'
,@date :=`reference_date` 'reference_date'
FROM
(SELECT * FROM datetable ORDER BY unique_identifier,reference_date) t1
,(SELECT @unique_identifier := 0) a,(SELECT @date := NOW()) b


Id
reference_date
60_days_since_seen
unique_identifier
reference_date


1
2020-01-05
1
123456789
2020-01-05

2
2020-02-20
0
123456789
2020-02-20

3
2020-06-29
1
123456789
2020-06-29

4
2020-02-21
1
987654321
2020-02-21

5
2020-03-30
0
987654321
2020-03-30


SELECT
`Id`, `unique_identifier`, `reference_date`,
   IF(DATEDIFF(reference_date , LAG(reference_date) OVER (
        PARTITION BY unique_identifier
        ORDER BY reference_date ))> 60 OR LAG(reference_date) OVER (
        PARTITION BY unique_identifier
        ORDER BY reference_date ) is NULL,1,0) 60_days_since_seen
FROM datetable


Id
unique_identifier
reference_date
60_days_since_seen


1
123456789
2020-01-05
1

2
123456789
2020-02-20
0

3
123456789
2020-06-29
1

4
987654321
2020-02-21
1

5
987654321
2020-03-30
0

Id	reference_date	60_days_since_seen	unique_identifier	reference_date
1	2020-01-05	1	123456789	2020-01-05
2	2020-02-20	0	123456789	2020-02-20
3	2020-06-29	1	123456789	2020-06-29
4	2020-02-21	1	987654321	2020-02-21
5	2020-03-30	0	987654321	2020-03-30