MySQL 5.7

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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);
CREATE FUNCTION PrevReading(sID int, rDate date)
RETURNS INT

BEGIN

   DECLARE val INT;

   SET val := (SELECT Reading
               FROM   Readings
               WHERE  SystemID = sID
                      AND ReadingDate < rDate
                      AND Reading IS NOT NULL
               ORDER BY 
                      SystemID, ReadingDate DESC
               LIMIT 1);

   RETURN val;

END; 
SELECT
    SystemID,
    ReadingDate,
    Reading,
    PrevReading(SystemID, ReadingDate),
    IF (Reading IS NULL, 0, Reading - PrevReading(SystemID, ReadingDate)) as Diff
FROM
    Readings
ORDER BY
    SystemID, ReadingDate;


SystemID
ReadingDate
Reading
PrevReading(SystemID, ReadingDate)
Diff


1
2021-02-01
24
null
null

1
2021-02-02
26
24
2

1
2021-02-03
23
26
-3

1
2021-02-04
null
23
0

1
2021-02-05
null
23
0

1
2021-02-06
28
23
5

1
2021-02-07
21
28
-7

2
2021-02-01
124
null
null

2
2021-02-02
126
124
2

2
2021-02-03
123
126
-3

2
2021-02-04
null
123
0

2
2021-02-05
null
123
0

2
2021-02-06
128
123
5

2
2021-02-07
121
128
-7


UPDATE
    Readings
SET
    Reading = IF(Reading IS NULL, PrevReading(SystemID, ReadingDate), Reading),
    Diff =    IF(Reading IS NULL, 0, Reading - PrevReading(SystemID, ReadingDate))
    
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	PrevReading(SystemID, ReadingDate)	Diff
1	2021-02-01	24	null	null
1	2021-02-02	26	24	2
1	2021-02-03	23	26	-3
1	2021-02-04	null	23	0
1	2021-02-05	null	23	0
1	2021-02-06	28	23	5
1	2021-02-07	21	28	-7
2	2021-02-01	124	null	null
2	2021-02-02	126	124	2
2	2021-02-03	123	126	-3
2	2021-02-04	null	123	0
2	2021-02-05	null	123	0
2	2021-02-06	128	123	5
2	2021-02-07	121	128	-7