SQL Server 2019

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select @@version;


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Microsoft SQL Server 2019 (RTM-CU16) (KB5011644) - 15.0.4223.1 (X64) 
 Apr 11 2022 16:24:07 
 Copyright (C) 2019 Microsoft Corporation
 Express Edition (64-bit) on Windows Server 2019 Standard 10.0 <X64> (Build 17763: ) (Hypervisor)



CREATE TABLE batches (
    Batch   VARCHAR(4)
  , Value   DECIMAL(5, 2)
  , RowID   int
  , GroupID int
);
INSERT INTO batches VALUES
  ('A' , 10.2 , 1 , 1 )
, ('A' ,  6.7 , 2 , 1 )
, ('A' ,  7.6 , 3 , 2 )
, ('B' ,  7.0 , 4 , 2 )
, ('B' , 10.2 , 5 , 3 )
, ('C' ,  9.6 , 6 , 3 )
, ('C' ,  8.2 , 7 , 4 )
, ('C' ,  1.1 , 8 , 4 )
, ('C' ,  0.3 , 9 , 5 )
, ('C' ,  9.0 ,10 , 5 )
, ('C' , 10.2 ,11 , 6 )
;
11 rows affected

WITH xrows AS (
       SELECT t.*
            , ROW_NUMBER() OVER (PARTITION BY Batch ORDER BY RowID) % 2 AS is_start
         FROM batches AS t
     )
SELECT t.*
     , SUM(is_start) OVER (ORDER BY Batch, RowID) AS new_GroupID
  FROM xrows AS t
 ORDER BY RowID
;


Batch
Value
RowID
GroupID
is_start
new_GroupID


A
10.20
1
1
1
1

A
6.70
2
1
0
1

A
7.60
3
2
1
2

B
7.00
4
2
1
3

B
10.20
5
3
0
3

C
9.60
6
3
1
4

C
8.20
7
4
0
4

C
1.10
8
4
1
5

C
0.30
9
5
0
5

C
9.00
10
5
1
6

C
10.20
11
6
0
6

Batch	Value	RowID	GroupID	is_start	new_GroupID
A	10.20	1	1	1	1
A	6.70	2	1	0	1
A	7.60	3	2	1	2
B	7.00	4	2	1	3
B	10.20	5	3	0	3
C	9.60	6	3	1	4
C	8.20	7	4	0	4
C	1.10	8	4	1	5
C	0.30	9	5	0	5
C	9.00	10	5	1	6
C	10.20	11	6	0	6