SQL Server running total with dbmail when total met or exceeded

I am able to get a running total via script--but what I can't seem to do is isolate the line where I have met or exceeded a certain value...

SELECT 
    column1, 
    (SELECT SUM(column1) FROM table WHERE column2 <= t1.column2)
FROM 
    table  t1 

Ultimately what I would want to do is create a trigger to send dbmail when the Sum of column1 meets or exceeds (n)...help me obi-wan

On SQL-Server 2008 you can use next solution:

DECLARE @TBL TABLE(id int, amount int);
INSERT INTO @TBL VALUES
(1, 100), (2, 100), (3, 60), (4, 200), (5, 100);

SELECT     t1.ID, t1.amount, SUM(t2.amount) as CumTotal
FROM       @TBL t1
CROSS APPLY (SELECT *
       FROM @TBL
       WHERE ID <= t1.id) t2
GROUP BY   t1.ID, t1.amount
HAVING     SUM(t1.amount) < 300
ORDER BY   t1.ID
;

This is the result:

ID | amount | CumTotal
-: | -----: | -------:
 1 |    100 |      100
 2 |    100 |      200
 3 |     60 |      260

dbfiddle here

Although this solution works well, it is not recommendable on terms of performance.

In this case the best option is to use a CURSOR.

DECLARE @CS table (id int, amount int, total int);
DECLARE @id int, @amount int;
DECLARE @CumSum int = 0;
DECLARE c CURSOR
    LOCAL STATIC FORWARD_ONLY READ_ONLY
    FOR SELECT ID, amount
        FROM @TBL
        ORDER BY [id];
 
OPEN c;
FETCH NEXT FROM c INTO @id, @amount
 
WHILE @@FETCH_STATUS = 0 AND @CumSum + @amount < 300 
BEGIN
    SET @CumSum = @CumSum + @amount;
    INSERT @CS (id, amount,  total)
    SELECT @id, @amount, @CumSum;
    FETCH NEXT FROM c INTO @id, @amount
END

CLOSE c;
DEALLOCATE c;
 
SELECT   id, amount, total
FROM     @CS
ORDER BY id;

GO

id | amount | total
-: | -----: | ----:
 1 |    100 |   100
 2 |    100 |   200
 3 |     60 |   260

dbfiddle here

Next answer can be used on SQL-SERVER 2012 and above

You can use a cumulative sum using a WINDOW function and SUM() ROWS UNBOUNDED PRECEDING.

Have a look at MS docs.

DECLARE @TBL TABLE(id int, amount int);
INSERT INTO @TBL VALUES
(1, 100), (2, 100), (3, 60), (4, 200), (5, 100);

Next query returns a cumulative sum:

SELECT ID, 
       SUM(amount) OVER (ORDER BY ID ROWS UNBOUNDED PRECEDING) AS CumTotal
FROM   @TBL
;

ID | CumTotal
-: | -------:
 1 |      100
 2 |      200
 3 |      260
 4 |      460
 5 |      560

The problem is that you cannot stop it, you need to calculate all records, and then you can apply a where clause just to filter the records.

WITH CSum As
(
    SELECT ID, 
           SUM(amount) OVER (ORDER BY ID ROWS UNBOUNDED PRECEDING) AS CumTotal
    FROM   @TBL
)
SELECT   ID, CumTotal
FROM     CSum
WHERE    CumTotal < 300
ORDER BY ID
;

This is the final result:

 
ID | CumTotal
-: | -------:
 1 |      100
 2 |      200
 3 |      260

dbfiddle here