Question

I am new to SQL in general and even newer to MS SQL. I apologize if the title isn’t clear on what I want.

I have two tables an old one that I want to derive data from into a new table. The tables have the exact same columns but different number of rows. The new table has multiple copies of each value in the old table, which only has 2 occurences. see below comparison of two columns: letter and amount.

New table:

A  0 
A  0
A  0
B  0
B  0

old table:

A  12
A  0
B  10
B  0
C  23

What I want to achieve is adding the values of the amount column from the old table into just the first occurence of the leter in the new table like so:

A  12 
A  0
A  0
B  10
B  0

Inner join causes all the values to be filled ( so all the A’s are set to 12).

Answer 1

try this:

DECLARE @test1 TABLE(col1 varchar(2),idn int)
insert into @test1
VALUES('A',0),
('A',0),
('A',0),
('B',0),
('B',0)

DECLARE @test2 TABLE(col1 varchar(2),idn int)
insert into @test2
VALUES('A',12),
('A',0),
('B',10),
('B',0),
('C',23)


;WITH CTE as (select *,ROW_NUMBER() over (partition by col1 order by col1) as rn from @test1)

update c SET c.idn=b.idn
from CTE c inner join (select col1,SUM(idn) as idn from @test2
group by col1) b
on c.col1 = b.col1 
where c.rn=1

select * from @test1