SQL Server 2019

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Microsoft SQL Server 2019 (RTM-CU3) (KB4538853) - 15.0.4023.6 (X64) 
 Mar  4 2020 00:59:26 
 Copyright (C) 2019 Microsoft Corporation
 Express Edition (64-bit) on Windows Server 2019 Standard 10.0 <X64> (Build 17763: ) (Hypervisor)



CREATE TABLE Sales (Id int IDENTITY(1,1) NOT NULL,stateid int, district int, Sitecount int)
CREATE TABLE Sales1(stateid int, district int, Sitecount Int)

insert into Sales values (1,2,12)
insert into Sales values (1,3,20)
insert into Sales values (1, NULL, 10) 
insert into Sales1 values (1,2,10) 
insert into Sales1 values (1,2, 100) 
insert into Sales1 values (1,NULL, 18)

select * from Sales
select * from Sales1


Id
stateid
district
Sitecount


1
1
2
12

2
1
3
20

3
1
null
10




stateid
district
Sitecount


1
2
10

1
2
100

1
null
18


MERGE Sales AS T 
USING (Select stateid, district, max(Sitecount) Sitecount from Sales1 group by stateid,district) as S
 ON(S.stateid =T.stateid and isnull(S.district,'')=isnull(T.district, '')) 
 WHEN MATCHED
Then UPDATE SET
T.Sitecount=T.Sitecount+S.Sitecount

WHEN NOT MATCHED BY TARGET THEN INSERT (stateid,district,Sitecount) VALUES(S.stateid, S.district, S.Sitecount);

select * from Sales


Id
stateid
district
Sitecount


1
1
2
112

2
1
3
20

3
1
null
28