SQL Server – How to Sum Values in Grouped Results

So really these tables aren't related in any way at all. Assuming you want each row in table 2 assigned to a row in table 1, here's one arbitrary way to do it:

;WITH t1 AS 
(
  SELECT ID, col_1, rn = ROW_NUMBER() OVER (ORDER BY col_1)
    FROM dbo.table_1
    WHERE ID = 11 -- guessing here based on sample data
),
t2 AS 
(
  SELECT col_2, rn = ROW_NUMBER() OVER (ORDER BY col_2) 
    FROM dbo.table_2
)
UPDATE t1 
    SET col_1 = t2.col_2
  FROM t1 
  INNER JOIN t2 
    ON t1.rn = t2.rn;

If you have other columns you want to order by, you can add them. If you only want three rows and there are more than three rows in table 2, add:

WHERE t2.rn <= 3;

Here's a recursive CTE example I came up with (which seems to work). Is uses Row_Number() OVER to create a sequence number with no gaps. I don't know how well it would perform with your data, but it's something to try.

--Set up demo data
IF OBJECT_ID('tempdb..#temp') IS NOT NULL
    drop table #temp
go
create table #temp (PKID int, NumValue int, GroupID int)
insert into #temp values
(1,-1,1), (3,-2,1), (5,5,1), (7,-3,1), (9,1,2)

--here is the real code
;
with RowNumberAddedToTemp as 
(
SELECT 
  ROW_NUMBER() OVER(ORDER BY PKID ASC) AS rn,
  * from #temp
  )
,x
AS (
    SELECT PKID         --Anchor row
        ,NumValue
        ,RunningTotal = CASE 
            WHEN NumValue < 0   --if initial value less than zero, make zero
                THEN 0
            ELSE NumValue
            END
        ,GroupID
        ,rn
    FROM RowNumberAddedToTemp
    WHERE rn = 1      

    UNION ALL

    SELECT y.PKID
        ,y.NumValue
        ,CASE 
            WHEN x.GroupID <> y.groupid     --did GroupId change?
                THEN CASE 
                        WHEN y.NumValue < 0     --if value is less than zero, make zero
                            THEN 0
                        ELSE y.numvalue         --start new groupid totals
                        END
            WHEN x.RunningTotal + y.NumValue < 0    --If adding the current row makes the total < 0, make zero
                THEN 0
            ELSE x.RunningTotal + y.NumValue        --Add to the running total for the current groupid
            END
        ,y.Groupid
        ,y.rn
    FROM x
    INNER JOIN RowNumberAddedToTemp AS y ON y.rn = x.rn + 1
    )
SELECT PKID
    ,Numvalue
    ,RunningTotal
    ,GroupID
FROM x
ORDER BY PKID
OPTION (MAXRECURSION 10000);