SQL Server 2019

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select @@version;


(No column name)


Microsoft SQL Server 2019 (RTM-CU6) (KB4563110) - 15.0.4053.23 (X64) 
	Jul 25 2020 11:26:55 
	Copyright (C) 2019 Microsoft Corporation
	Express Edition (64-bit) on Windows Server 2019 Standard 10.0 <X64> (Build 17763: ) (Hypervisor)



DROP TABLE if exists mytable;
CREATE TABLE mytable(Process char(1), Client char(1));

INSERT INTO mytable values
  ('A',null),
  ('A','B'),
  ('A','B'),
  ('A','B'),
  ('A','C'),
  ('A','C'),
  ('A','C');
7 rows affected

-- With a NULL value
select 
   Process, 
   Client, 
   ROW_NUMBER() OVER (partition by process,Client order by (select null))+CASE WHEN Client is null THEN 0 else 1 end R 
from mytable;


Process
Client
R


A
null
1

A
B
2

A
B
3

A
B
4

A
C
2

A
C
3

A
C
4


-- without a NULL value
select 
   Process, 
   Client, 
   ROW_NUMBER() OVER (partition by process,COALESCE(Client,'B') order by (select null)) R 
from mytable
where not client is null;


Process
Client
R


A
B
1

A
B
2

A
B
3

A
C
1

A
C
2

A
C
3