Update a column based on the value on another column

I have an SQL table dbo.aTest. I want to update Col3Update if Col1 is AA and whatever value in Col2, copy that value in Col3Update from Col1 - values AA,BB,CC,DD,EE and next Col1 that starts with AA again. See attached.

I have a Cursor here that was answered by Ryan Chong. Thank you again Ryan Chong.

declare @uniqueid int;
declare @column2 nvarchar(100);
declare @last_column2 nvarchar(100);
declare @cnt int;

declare cur cursor for  
select uniqueid, [col2]
from dbo.aTest
where  [col1] = 'AA'  
order by uniqueid

open cur  
fetch next from cur into @uniqueid, @column2

set @last_column2 = ''
set @cnt = 0

while @@fetch_status = 0  
begin  
       if @last_column2 = '' or (len(@column2)> 1 and @cnt > 1)
			set @last_column2 = @column2
			
	   update a 
	   set Col3Update = @last_column2 
	   from dbo.aTest a
	   where uniqueid = @uniqueid
	   set @cnt = @cnt + 1
	   
       fetch next from cur into @uniqueid, @column2  
end 

close cur;
deallocate cur;

select * from dbo.aTest
order by uniqueid

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What I did wrong in this cursor?

Please help.

Thank you again for all your help.
SQL_and_OUTPUT_NUMBERS.xlsx

Queennie LAsked: 2018-03-01

Who is Participating?

LVL 71

Scott PletcherSenior DBACommented: 2018-03-02

D'OH, sorry. I didn't sort correctly in the OUTER APPLY:

OUTER APPLY (
SELECT TOP (1) row_num AS Col3Update
FROM cte_base_column cbc
WHERE a.UniqueId >= cbc.UniqueId
ORDER BY UniqueId DESC
) AS oa1

LVL 71

Scott PletcherSenior DBACommented: 2018-03-01

I don't see any need for a cursor. I don't have actual, usable data to test with, but something like this should do it:

;WITH cte_base_column AS (
SELECT UniqueId, ROW_NUMBER() OVER(ORDER BY UniqueId) AS row_num
FROM dbo.aTest
WHERE col1 = 'AA'
)
SELECT a.UniqueId, a.Col1, a.Col2, oa1.Col3Update
FROM dbo.aTest a
OUTER APPLY (
SELECT TOP (1) row_num AS Col3Update
FROM cte_base_column cbc
WHERE a.UniqueId >= cbc.UniqueId
ORDER BY UniqueId
) AS oa1

Queennie LAuthor Commented: 2018-03-02

@ScottPletcher:

When the above query, this is the output

UniqueId      Col1      Col2      Col3Update
1      AA      1      1
2      BB      11111111111      1
3      CC      472      1
4      DD      AD      1
5      EE      6      1
6      AA      2      1
7      BB      22222222222      1
8      CC      472      1
9      DD      OA      1
10      EE      6      1

Thank you for your help.