MySQL 8.0

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select version();


version()


8.0.22


create table test (LogDateAndTime timestamp default null, BatchDate timestamp default null, TagLetter varchar(10), Totaliser bigint);
insert into test values('2020-11-10 09:06:14', '2020-11-10 08:29:55', 'A',6319);
insert into test values('2020-11-10 09:06:24', '2020-11-10 08:29:55', 'A',6337);
insert into test values('2020-11-10 09:08:14', '2020-11-10 08:29:55', 'B',6355);
insert into test values('2020-11-10 09:08:24', '2020-11-10 08:29:55', 'B',6372);
insert into test values('2020-11-10 09:08:34', '2020-11-10 08:29:55', 'B',6378);
insert into test values('2020-11-10 09:08:44', '2020-11-10 08:29:55', 'A',6383);
insert into test values('2020-11-10 09:09:14', '2020-11-10 08:29:55', 'A',6388);
insert into test values('2020-11-10 09:09:24', '2020-11-10 08:29:55', 'A',6396);
insert into test values('2020-11-10 09:09:34', '2020-11-10 08:29:55', 'B',6409);
insert into test values('2020-11-10 09:09:44', '2020-11-10 08:29:55', 'B',6426);
insert into test values('2020-11-10 09:10:24', '2020-11-10 08:29:55', 'B',6442);
with cte as (
select 
test.*, 
@rn := (if (@rt =TagLetter, @rn, @rn+1)) rank_,
@rt :=TagLetter
from test , (select @rn := 1,  @rt := '') t
)

select t1.*,t2.ExpectedResult from test t1
left join (
select  distinct BatchDate, sum(CASE 
             WHEN TagLetter='A' THEN MAX(Totaliser)-MIN(Totaliser) 
             ELSE 0.0 
           END) over ()
           AS ExpectedResult
          
          from cte
          
group by BatchDate,TagLetter,rank_) t2 on t1.BatchDate=t2.BatchDate


LogDateAndTime
BatchDate
TagLetter
Totaliser
ExpectedResult


2020-11-10 09:06:14
2020-11-10 08:29:55
A
6319
31.0

2020-11-10 09:06:24
2020-11-10 08:29:55
A
6337
31.0

2020-11-10 09:08:14
2020-11-10 08:29:55
B
6355
31.0

2020-11-10 09:08:24
2020-11-10 08:29:55
B
6372
31.0

2020-11-10 09:08:34
2020-11-10 08:29:55
B
6378
31.0

2020-11-10 09:08:44
2020-11-10 08:29:55
A
6383
31.0

2020-11-10 09:09:14
2020-11-10 08:29:55
A
6388
31.0

2020-11-10 09:09:24
2020-11-10 08:29:55
A
6396
31.0

2020-11-10 09:09:34
2020-11-10 08:29:55
B
6409
31.0

2020-11-10 09:09:44
2020-11-10 08:29:55
B
6426
31.0

2020-11-10 09:10:24
2020-11-10 08:29:55
B
6442
31.0

LogDateAndTime	BatchDate	TagLetter	Totaliser	ExpectedResult
2020-11-10 09:06:14	2020-11-10 08:29:55	A	6319	31.0
2020-11-10 09:06:24	2020-11-10 08:29:55	A	6337	31.0
2020-11-10 09:08:14	2020-11-10 08:29:55	B	6355	31.0
2020-11-10 09:08:24	2020-11-10 08:29:55	B	6372	31.0
2020-11-10 09:08:34	2020-11-10 08:29:55	B	6378	31.0
2020-11-10 09:08:44	2020-11-10 08:29:55	A	6383	31.0
2020-11-10 09:09:14	2020-11-10 08:29:55	A	6388	31.0
2020-11-10 09:09:24	2020-11-10 08:29:55	A	6396	31.0
2020-11-10 09:09:34	2020-11-10 08:29:55	B	6409	31.0
2020-11-10 09:09:44	2020-11-10 08:29:55	B	6426	31.0
2020-11-10 09:10:24	2020-11-10 08:29:55	B	6442	31.0