SQL Server 2017

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CREATE TABLE aa (
  a1 varchar(20), -- Time
  a2 float        -- Total_Act_Energy
);

INSERT INTO aa
VALUES ('2021-05-10 18:15', 425.8),
  ('2021-05-10 19:15', 525.8),
  ('2021-05-10 20:15', 625.8),
  ('2021-05-11 18:15', 427.1),
  ('2021-05-11 19:15', 527.1),
  ('2021-05-11 20:15', 627.1),
  ('2021-05-11 21:15', 727.1),
  ('2021-05-12 18:16', 428.3),
  ('2021-05-12 19:16', 528.3),
  ('2021-05-13 09:47', 428.7);

SELECT *
FROM aa;


a1
a2


2021-05-10 18:15
425.8

2021-05-10 19:15
525.8

2021-05-10 20:15
625.8

2021-05-11 18:15
427.1

2021-05-11 19:15
527.1

2021-05-11 20:15
627.1

2021-05-11 21:15
727.1

2021-05-12 18:16
428.3

2021-05-12 19:16
528.3

2021-05-13 09:47
428.7


CREATE TABLE bb (
  b1 varchar(20),
  b2 varchar(20),
  b3 float,
  b4 float
);
INSERT INTO bb (b1, b2, b3, b4)
SELECT a1,a3,a2,ROUND(a2 - LAG(a2, 1, a2) OVER (ORDER BY a1), 2) AS a4
FROM (
  SELECT a1,CONVERT(varchar(10), a1, 120) AS a3,a2
  FROM (
    SELECT *,ROW_NUMBER() OVER (PARTITION BY CONVERT(varchar(10), a1, 120) ORDER BY a1 DESC) AS roworder
    FROM aa) AS m
  WHERE roworder = 1) AS k
ORDER BY a1
4 rows affected

SELECT *
FROM bb
ORDER BY b1


b1
b2
b3
b4


2021-05-10 20:15
2021-05-10
625.8
0

2021-05-11 21:15
2021-05-11
727.1
101.3

2021-05-12 19:16
2021-05-12
528.3
-198.8

2021-05-13 09:47
2021-05-13
428.7
-99.6

a1	a2
2021-05-10 18:15	425.8
2021-05-10 19:15	525.8
2021-05-10 20:15	625.8
2021-05-11 18:15	427.1
2021-05-11 19:15	527.1
2021-05-11 20:15	627.1
2021-05-11 21:15	727.1
2021-05-12 18:16	428.3
2021-05-12 19:16	528.3
2021-05-13 09:47	428.7

b1	b2	b3	b4
2021-05-10 20:15	2021-05-10	625.8	0
2021-05-11 21:15	2021-05-11	727.1	101.3
2021-05-12 19:16	2021-05-12	528.3	-198.8
2021-05-13 09:47	2021-05-13	428.7	-99.6