SQL Group By Modulo of Row Count - sql

Question

I have the following sample data:
Id Name Quantity
1 Red 1
2 Red 3
3 Blue 1
4 Red 1
5 Yellow 3
So for this example, there are a total of 5 Red, 1 Blue, and 3 Yellow. I am looking for a way to group them by Color, but with a maximum of 2 items per group (sorting is not important). Like so:
Name QuantityInPackage
Red 2
Red 2
Red 1
Blue 1
Yellow 2
Yellow 1
Any suggestions on how to accomplish this using T-SQL on MS-SQL 2005?

Answer 1

I would define a table containing sequential numbers, say 1 to 1000 and join that table (unless your database supports generating these numbers in the query like Oracle using CONNECT BY):
Table num
n
1
2
3
...
I tried the following query using Oracle (should work with TSQL too):
With summed_colors As (
Select name, Sum(quantity) quantity
From colors
Group By name
)
Select
name,
Case When n*2-1 = quantity Then 1 Else 2 End quantityInPackage
From summed_colors
Join nums On ( n*2-1 <= quantity )
Order By name, quantityInPackage Desc
and it returns
Blue 1
Red 2
Red 2
Red 1
Yellow 2
Yellow 1

Answer 2

You need to use a numbers table to unpivot your data to make multiple rows:
DECLARE #PackageSize AS int
SET #PackageSize = 2
DECLARE #numbers AS TABLE (Number int)
INSERT INTO #numbers
VALUES (1)
INSERT INTO #numbers
VALUES (2)
INSERT INTO #numbers
VALUES (3)
INSERT INTO #numbers
VALUES (4)
INSERT INTO #numbers
VALUES (5)
INSERT INTO #numbers
VALUES (6)
INSERT INTO #numbers
VALUES (7)
INSERT INTO #numbers
VALUES (8)
INSERT INTO #numbers
VALUES (9)
INSERT INTO #numbers
VALUES (10)
DECLARE #t AS TABLE
(
Id int
,Nm varchar(6)
,Qty int
)
INSERT INTO #t
VALUES (1, 'Red', 1)
INSERT INTO #t
VALUES (2, 'Red', 3)
INSERT INTO #t
VALUES (3, 'Blue', 1)
INSERT INTO #t
VALUES (4, 'Red', 1)
INSERT INTO #t
VALUES (5, 'Yellow', 3) ;
WITH Totals
AS (
SELECT Nm
,SUM(Qty) AS TotalQty
,SUM(Qty) / #PackageSize AS NumCompletePackages
,SUM(Qty) % #PackageSize AS PartialPackage
FROM #t
GROUP BY Nm
)
SELECT Totals.Nm
,#PackageSize AS QuantityInPackage
FROM Totals
INNER JOIN #numbers AS numbers
ON numbers.Number <= Totals.NumCompletePackages
UNION ALL
SELECT Totals.Nm
,PartialPackage AS QuantityInPackage
FROM Totals
WHERE PartialPackage <> 0

Answer 3

It's not grouping or modulo/division that's the hard part here, it's the fact that you need to do an aggregate (sum) and then explode the data again. There aren't actually any "Red 2" rows, you have to create them somehow.
For SQL Server 2005+, I would probably use a function do the "exploding":
CREATE FUNCTION dbo.CreateBuckets
(
#Num int,
#MaxPerGroup int
)
RETURNS TABLE
AS RETURN
WITH First_CTE AS
(
SELECT CASE
WHEN #MaxPerGroup < #Num THEN #MaxPerGroup
ELSE #Num
END AS Seed
),
Sequence_CTE AS
(
SELECT Seed AS [Current], Seed AS Total
FROM First_CTE
UNION ALL
SELECT
CASE
WHEN (Total + #MaxPerGroup) > #Num THEN (#Num - Total)
ELSE #MaxPerGroup
END,
Total + #MaxPerGroup
FROM Sequence_CTE
WHERE Total < #Num
)
SELECT [Current] AS Num
FROM Sequence_CTE
Then, in the main query, group (sum) the data first and then use the bucket function:
WITH Totals AS
(
SELECT Name, SUM(Quantity) AS Total
FROM Table
GROUP BY Name
)
SELECT Name, b.Num AS QuantityInPackage
FROM Totals
CROSS APPLY dbo.CreateBuckets(Total, 2) b
This should work for any bucket size, doesn't have to be 2 (just change the parameter).

Answer 4

This is very crude, but it works.
CREATE TABLE #Colors
(
Id int,
Name varchar(50),
Quantity int
)
INSERT INTO #Colors VALUES (1, 'Red', 1)
INSERT INTO #Colors VALUES (2, 'Red', 3)
INSERT INTO #Colors VALUES (3, 'Blue', 1)
INSERT INTO #Colors VALUES (4, 'Red', 1)
INSERT INTO #Colors VALUES (5, 'Yellow', 3)
INSERT INTO #Colors VALUES (6, 'Green', 2)
SELECT
Name,
SUM(Quantity) AS TotalQuantity
INTO #Summed
FROM
#Colors
GROUP BY
Name
SELECT
Name,
TotalQuantity / 2 AS RecordsWithQuantity2,
TotalQuantity % 2 AS RecordsWithQuantity1
INTO #SortOfPivot
FROM
#Summed
ORDER BY
Name
DECLARE #RowCount int
SET #RowCount = (SELECT COUNT(*) FROM #SortOfPivot)
DECLARE #Name varchar(50)
DECLARE #TwosInsertCount int
DECLARE #OnesInsertCount int
CREATE TABLE #Result (Name varchar(50), Quantity int)
WHILE #RowCount > 0
BEGIN
SET #Name = (SELECT TOP 1 Name FROM #SortOfPivot)
SET #TwosInsertCount = (SELECT TOP 1 RecordsWithQuantity2 FROM #SortOfPivot)
SET #OnesInsertCount = (SELECT TOP 1 RecordsWithQuantity1 FROM #SortOfPivot)
WHILE #TwosInsertCount > 0
BEGIN
INSERT INTO #Result (Name, Quantity) VALUES (#Name, 2)
SET #TwosInsertCount = #TwosInsertCount - 1
END
WHILE #OnesInsertCount > 0
BEGIN
INSERT INTO #Result (Name, Quantity) VALUES (#Name, 1)
SET #OnesInsertCount = #OnesInsertCount - 1
END
DELETE FROM #SortOfPivot WHERE Name = #Name
SET #RowCount = (SELECT COUNT(*) FROM #SortOfPivot)
END
SELECT * FROM #Result
DROP TABLE #Colors
DROP TABLE #Result
DROP TABLE #Summed
DROP TABLE #SortOfPivot