Split a quantity into multiple rows with limit on quantity per row - sql

Question

I have a table of ids and quantities that looks like this:
dbo.Quantity
id | qty
-------
1 | 3
2 | 6
I would like to split the quantity column into multiple lines and number them, but with a set limit (which can be arbitrary) on the maximum quantity allowed for each row.
So for the value of 2, expected output should be:
dbo.DesiredResult
id | qty | bucket
---------------
1 | 2 | 1
1 | 1 | 2
2 | 1 | 2
2 | 2 | 3
2 | 2 | 4
2 | 1 | 5
In other words,
Running SELECT id, SUM(qty) as qty FROM dbo.DesiredResult should return the original table (dbo.Quantity).
Running
SELECT id, SUM(qty) as qty FROM dbo.DesiredResult GROUP BY bucket
should give you this table.
id | qty | bucket
------------------
1 | 2 | 1
1 | 2 | 2
2 | 2 | 3
2 | 2 | 4
2 | 1 | 5
I feel I can do this with cursors imperitavely, looping over each row, keeping a counter that increments and resets as the "max" for each is filled. But this is very "anti-SQL" I feel there is a better way around this.

Answer 1

One approach is recursive CTE which emulates cursor sequentially going through rows.
Another approach that comes to mind is to represent your data as intervals and intersections of intervals.
Represent this:
id | qty
-------
1 | 3
2 | 6
as intervals [0;3), [3;9) with ids being their labels
0123456789
|--|-----|
1 2 - id
It is easy to generate this set of intervals using running total SUM() OVER().
Represent your buckets also as intervals [0;2), [2;4), [4;6), etc. with their own labels
0123456789
|-|-|-|-|-|
1 2 3 4 5 - bucket
It is easy to generate this set of intervals using a table of numbers.
Intersect these two sets of intervals preserving information about their labels.
Working with sets should be possible in a set-based SQL query, rather than a sequential cursor or recursion.
It is bit too much for me to write down the actual query right now. But, it is quite possible that ideas similar to those discussed in Packing Intervals by Itzik Ben-Gan may be useful here.
Actually, once you have your quantities represented as intervals you can generate required number of rows/buckets on the fly from the table of numbers using CROSS APPLY.
Imagine we transformed your Quantity table into Intervals:
Start | End | ID
0 | 3 | 1
3 | 9 | 2
And we also have a table of numbers - a table Numbers with column Number with values from 0 to, say, 100K.
For each Start and End of the interval we can calculate the corresponding bucket number by dividing the value by the bucket size and rounding down or up.
Something along these lines:
SELECT
Intervals.ID
,A.qty
,A.Bucket
FROM
Intervals
CROSS APPLY
(
SELECT
Numbers.Number + 1 AS Bucket
,#BucketSize AS qty
-- it is equal to #BucketSize if the bucket is completely within the Start and End boundaries
-- it should be adjusted for the first and last buckets of the interval
FROM Numbers
WHERE
Numbers.Number >= Start / #BucketSize
AND Numbers.Number < End / #BucketSize + 1
) AS A
;
You'll need to check and adjust formulas for errors +-1.
And write some CASE WHEN logic for calculating the correct qty for the buckets that happen to be on the lower and upper boundary of the interval.

Answer 2

Use a recursive CTE:
with cte as (
select id, 1 as n, qty
from t
union all
select id, n + 1, qty
from cte
where n + 1 < qty
)
select id, n
from cte;
Here is a db<>fiddle.