T-SQL query: find most frequent pair of values - sql

Question

I have text column with numeric values separated by semicolons. I'm trying to figure out how to get the most frequent pair of values that appeared together in the same row. I've found a solution for a very similar problem in Python Finding the most frequent occurrences of pairs in a list of lists, but I don't know how to rewrite it in using SQL In example below it returns 2 and 3 because this pair appeared 3 times in the input set:
Input rows Output
---------- -------
';1;2;3;5' | '2;3'
';2;3' | '1;2'
';3;4;5;1;2' | '1;3'
';1;5;2' | '1;5'
Orginal data:

Answer 1

You may try with the following approach. First, using OPENJSON(), get all possible combinations. When OPENJSON parses a JSON array the indexes of the elements in the JSON text are returned as keys (0-based). Then, count the most frequent pair with DENSE_RANK().
Input:
CREATE TABLE #Items (
Id int,
ItemValues varchar(max)
)
INSERT INTO #Items
(Id, ItemValues)
VALUES
(1, '1;2;3;5'),
(2, '2;3'),
(3, '3;4;5;1;2'),
(4, '1;5;2')
Statement:
;WITH combinationsCTE AS (
SELECT
CASE
WHEN s1.[value] <= s2.[value] THEN CONCAT(s1.[value], ';', s2.[value])
ELSE CONCAT(s2.[value], ';', s1.[value])
END AS PairValue
FROM #Items i
CROSS APPLY (SELECT [key], [value] FROM OPENJSON('["' + REPLACE(i.ItemValues,';','","') + '"]')) s1
CROSS APPLY (SELECT [key], [value] FROM OPENJSON('["' + REPLACE(i.ItemValues,';','","') + '"]')) s2
WHERE (s1.[key] < s2.[key])
), rankingCTE AS (
SELECT
PairValue,
DENSE_RANK() OVER (ORDER BY COUNT(PairValue) DESC) AS PairRank
FROM combinationsCTE
GROUP BY PairValue
)
SELECT PairValue
FROM rankingCTE
WHERE PairRank = 1
Output:
PairValue
1;2
1;5
2;3
2;5

Answer 2

First have a split function
CREATE FUNCTION Splitfn(#String varchar(8000), #Delimiter char(1))
returns #temptable TABLE (items varchar(8000))
as
begin
declare #idx int
declare #slice varchar(8000)
select #idx = 1
if len(#String)<1 or #String is null return
while #idx!= 0
begin
set #idx = charindex(#Delimiter,#String)
if #idx!=0
set #slice = left(#String,#idx - 1)
else
set #slice = #String
if(len(#slice)>0)
insert into #temptable(Items) values(#slice)
set #String = right(#String,len(#String) - #idx)
if len(#String) = 0 break
end
return
end
Second Step To get all rows in a single string
Declare #val Varchar(MAX);
Select #val = COALESCE(#val + '; ' + YourColumn, YourColumn)
From YourTable
Third step,
SELECT TOP 1 items, count(*)
FROM dbo.Splitfn(#Val, ';')
WHERE LTRIM(RTRIM(items)) <> ''
GROUP BY items
ORDER BY Count(*) DESC