Question

I have a Total value that I need to distribute among several rows in a SQL table:

DECLARE @total numeric(38,5);
DECLARE @count int;

SET @total=123.10000

SET @count = SELECT COUNT(*) FROM mytable WHERE condition=@val;
-- let's say @count is now 3

UPDATE mytable SET my_part=@total/@count WHERE condition=@val;

--each record now has 41.03333

SELECT SUM(my_part) FROM mytable where condition = @val;

-- the sum is 123.09999, not my original 123.10000

Obviously, the original total wasn’t evenly divisible by 3 so the SUM won’t match the original value. And no matter what I use for scale, there will be possible divisions like this one that can’t line back up.

What I would like is that one of the UPDATEd rows would contain 41.03334, and the other two would have 41.03333. I don’t care which ones round up and which round down. But I care that the values can be re-summed to get the original total. Is this possible? Are there known algorithms for doing this kind of thing?

Answer 1

You could use fractions to avoid rounding problems. At least multiplication and division of several rows would be easy. SUM() would not be quite so easy, if you need the exact value.