Postgres 14

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create table tbl (fid int, result int, time int);
insert into tbl values
(1, 10, 100),
(1, 9, 85),
(2, 9, 92),
(2, 9, 99),
(3, 8, 88),
(3, 7, 100),
(3, 8, 67),
(4, 5, 50),
(4, 5, 50)
9 rows affected

SELECT fid, time, 
time  - LAG(time, 1) OVER (PARTITION BY fid ORDER BY time) as Difference_Between_Times
FROM tbl




fid
time
difference_between_times


1
85
null

1
100
15

2
92
null

2
99
7

3
67
null

3
88
21

3
100
12

4
50
null

4
50
0

fid	time	difference_between_times
1	85	null
1	100	15
2	92	null
2	99	7
3	67	null
3	88	21
3	100	12
4	50	null
4	50	0