How do I count unique combinations from a detail table? - sql

Question

I need to write a query in SQL to count the number of unique combinations of record. I have a table of items with a child table listing options for each item. Each item may have 0 to x number of options. I want to count how many of each combinations there are. I thought I could take the child table and transpose it using pivot and unpivot, but I haven't figured it out. I then tried creating a list of the combinations, but I don't know how to count the occurrences. Can someone show me how to do this or point me in the right direction?
Here is the table I want to use:
Item | Option
----------------
1 | A
1 | B
2 | B
3 | B
4 | B
4 | C
5 | A
6 | A
6 | B
6 | C
7 | A
7 | B
7 | C
8 | A
8 | B
9 | A
10 | A
10 | B
The results I want are this:
Option 1 | Option 2 | Option 3 | Count
--------------------------------------------
A | B | | 3 * 1, 8, 10
B | | | 2 * 2, 3
B | C | | 1 * 4
A | | | 2 * 5, 9
A | B | C | 2 * 6, 7
This is saying that the combination A and B occurred twice, twice B was the only option picked, B and C were picked together 1 time. (The numbers after the asterisk aren't part of the result, they're just there to show which items are being counted.)
The closest I've come is the query below. It gives me the unique combinations, but doesn't tell me how many times that combination occurred:
SELECT ItemCombo, Count(*) AS ItemComboCount
FROM
(
SELECT
Item
,STUFF((SELECT ',' + CAST(Option AS varchar(MAX))
FROM itemDetail a
WHERE a.Item = b.Item
FOR XML PATH(''), TYPE).value('.', 'VARCHAR(MAX)'),1,1,''
) AS ItemCombo
FROM itemDetail b
) AS Combos
GROUP BY ItemCombo
ORDER BY Count(*) DESC

Answer 1

You should group by in the inner query and also order by option so the concatenated values can be correctly grouped.
SELECT ItemCombo, Count(*) AS ItemComboCount
FROM
(
SELECT
Item
,STUFF((SELECT ',' + CAST(Option AS varchar(MAX))
FROM itemDetail a
WHERE a.Item = b.Item
ORDER BY Option
FOR XML PATH(''), TYPE).value('.', 'VARCHAR(MAX)'),1,1,''
) AS ItemCombo
FROM itemDetail b
GROUP BY item
) AS Combos
GROUP BY ItemCombo
ORDER BY Count(*) DESC

Answer 2

To address the additional requirement you mentioned in the comments I would add a CTE, some more XML processing and dynamic TSQL to Vamsi Prabhala's excellent answer (+1 from my side):
--create test table
create table tmp (Item int, [Option] char(1))
--populate test table
insert into tmp values ( 1, 'A') ,( 1, 'B') ,( 2, 'B') ,( 3, 'B') ,( 4, 'B') ,( 4, 'C') ,( 5, 'A') ,( 6, 'A') ,( 6, 'B') ,( 6, 'C') ,( 7, 'A') ,( 7, 'B') ,( 7, 'C') ,( 8, 'A') ,( 8, 'B') ,( 9, 'A') ,(10, 'A') ,(10, 'B')
declare #count int
declare #loop int = 1
declare #dynamicColums nvarchar(max) = ''
declare #sql nvarchar(max) = ''
--count possible values
select #count = max(c.options_count) from (
select count(*) as options_count from tmp group by item
) c
--build dynamic headers for all combinations
while #loop <= #count
begin
set #dynamicColums = #dynamicColums + ' Parts.value(N''/x['+ cast(#loop as nvarchar(max)) +']'', ''char(1)'') AS [Option ' + cast(#loop as nvarchar(max)) + '],'
set #loop = #loop + 1
end
--build dynamic TSQL statement
set #sql = #sql + ';WITH Splitted'
set #sql = #sql + ' AS ('
set #sql = #sql + ' SELECT ItemComboCount'
set #sql = #sql + ' ,ItemCombo'
set #sql = #sql + ' ,CAST(''<x>'' + REPLACE(ItemCombo, '','', ''</x><x>'') + ''</x>'' AS XML) AS Parts'
set #sql = #sql + ' FROM '
set #sql = #sql + ' ('
set #sql = #sql + ' SELECT ItemCombo, Count(*) AS ItemComboCount'
set #sql = #sql + ' FROM'
set #sql = #sql + ' ('
set #sql = #sql + ' SELECT'
set #sql = #sql + ' Item '
set #sql = #sql + ' ,STUFF((SELECT '','' + CAST([Option] AS varchar(MAX))'
set #sql = #sql + ' FROM tmp a '
set #sql = #sql + ' WHERE a.Item = b.Item'
set #sql = #sql + ' ORDER BY [Option]'
set #sql = #sql + ' FOR XML PATH(''''), TYPE).value(''.'', ''VARCHAR(MAX)''),1,1,'''''
set #sql = #sql + ' ) AS ItemCombo'
set #sql = #sql + ' FROM tmp b'
set #sql = #sql + ' GROUP BY item'
set #sql = #sql + ' ) AS Combos'
set #sql = #sql + ' GROUP BY ItemCombo'
set #sql = #sql + ' ) t'
set #sql = #sql + ' )'
set #sql = #sql + ' SELECT '
set #sql = #sql + #dynamicColums
set #sql = #sql + ' ItemComboCount as [Count]'
set #sql = #sql + ' FROM Splitted'
--execute dynamic TSQL statement
exec(#sql)
Results:
Now if you add another value (for example 'D') with a couple of insert statements:
insert into tmp values ( 1, 'D')
insert into tmp values ( 7, 'D')
you'll see that new columns are dinamically generated: