MySQL 8.0

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CREATE TABLE Readings (`SystemID` INT, `ReadingDate` DATE, `Reading` INT NULL, `Diff` INT NULL, PRIMARY KEY(`SystemID`,`ReadingDate`));

INSERT INTO Readings VALUES 
(1, '2021-02-01', 24, NULL), 
(1, '2021-02-02', 26, NULL), 
(1, '2021-02-03', 23, NULL), 
(1, '2021-02-04', NULL, NULL), 
(1, '2021-02-05', NULL, NULL), 
(1, '2021-02-06', 28, NULL), 
(1, '2021-02-07', 21, NULL),
(2, '2021-02-01', 124, NULL), 
(2, '2021-02-02', 126, NULL), 
(2, '2021-02-03', 123, NULL), 
(2, '2021-02-04', NULL, NULL), 
(2, '2021-02-05', NULL, NULL), 
(2, '2021-02-06', 128, NULL), 
(2, '2021-02-07', 121, NULL);
SELECT
    Act.SystemID,
    Act.ReadingDate,
    Act.Reading,
    Act.Diff,
    Prev.Reading as PrevReading
FROM
    Readings AS Act,
LATERAL
    (SELECT   Reading
     FROM     Readings
     WHERE    SystemID = Act.SystemID
              AND ReadingDate < Act.ReadingDate
              AND Reading IS NOT NULL
     ORDER BY SystemID, ReadingDate DESC
     LIMIT 1) Prev
     
ORDER BY
    Act.SystemID, Act.ReadingDate;


SystemID
ReadingDate
Reading
Diff
PrevReading


1
2021-02-02
26
null
24

1
2021-02-03
23
null
26

1
2021-02-04
null
null
23

1
2021-02-05
null
null
23

1
2021-02-06
28
null
23

1
2021-02-07
21
null
28

2
2021-02-02
126
null
124

2
2021-02-03
123
null
126

2
2021-02-04
null
null
123

2
2021-02-05
null
null
123

2
2021-02-06
128
null
123

2
2021-02-07
121
null
128


UPDATE
    Readings Act,
    LATERAL
    (SELECT   Reading
     FROM     Readings 
     WHERE    SystemID = Act.SystemID
              AND ReadingDate < Act.ReadingDate
              AND Reading IS NOT NULL
     ORDER BY SystemID, ReadingDate DESC
     LIMIT 1) Prev    
SET
    Act.Reading = IF(Act.Reading IS NULL, Prev.Reading, Act.Reading),

    Act.Diff = CASE 
                   WHEN Act.Reading IS NULL AND Prev.Reading IS NULL THEN 0
                   WHEN Prev.Reading IS NOT NULL THEN COALESCE(Act.Reading - Prev.Reading, 0)
                   ELSE Act.Reading - Prev.Reading END;
           
SELECT * FROM Readings ORDER BY SystemID, ReadingDate;           


SystemID
ReadingDate
Reading
Diff


1
2021-02-01
24
null

1
2021-02-02
26
2

1
2021-02-03
23
-3

1
2021-02-04
23
0

1
2021-02-05
23
0

1
2021-02-06
28
5

1
2021-02-07
21
-7

2
2021-02-01
124
null

2
2021-02-02
126
2

2
2021-02-03
123
-3

2
2021-02-04
123
0

2
2021-02-05
123
0

2
2021-02-06
128
5

2
2021-02-07
121
-7

SystemID	ReadingDate	Reading	Diff	PrevReading
1	2021-02-02	26	null	24
1	2021-02-03	23	null	26
1	2021-02-04	null	null	23
1	2021-02-05	null	null	23
1	2021-02-06	28	null	23
1	2021-02-07	21	null	28
2	2021-02-02	126	null	124
2	2021-02-03	123	null	126
2	2021-02-04	null	null	123
2	2021-02-05	null	null	123
2	2021-02-06	128	null	123
2	2021-02-07	121	null	128

SystemID	ReadingDate	Reading	Diff
1	2021-02-01	24	null
1	2021-02-02	26	2
1	2021-02-03	23	-3
1	2021-02-04	23	0
1	2021-02-05	23	0
1	2021-02-06	28	5
1	2021-02-07	21	-7
2	2021-02-01	124	null
2	2021-02-02	126	2
2	2021-02-03	123	-3
2	2021-02-04	123	0
2	2021-02-05	123	0
2	2021-02-06	128	5
2	2021-02-07	121	-7