Split string and fold in T-SQL - sql

Question

Is it possible to split a delimited string and then 'fold' the delimited parts such that the result is a string containing all possible 'paths'? I'm looking to purely use built-in functions if possible without resorting to recursive CTEs, etc.
This is a common functional pattern known as scan/fold. Wondering if T-SQL has a similar pattern.
Example
FOLD('A|B|C|D') = '[A],[A|B],[A|B|C],[A|B|C|D]'
EDIT: The order of the substrings must remain the same in the result. The target SQL version is Azure SQL.

Answer 1

if you have sql-server-2017 you can use STRING_AGG and STRING_SPLIT
declare #text VARCHAR(MAX) ='A|B|C|D'
declare #result VARCHAR(MAX) = ''
SELECT #result = #result + ',[' + STRING_AGG(X.value, '|') + ']' FROM
STRING_SPLIT(#text ,'|') X
INNER JOIN STRING_SPLIT(#text ,'|') Y
ON X.value <= Y.Value
GROUP BY Y.Value
SET #result = STUFF(#result,1,1,'')
print #result
Result:
[A],[A|B],[A|B|C],[A|B|C|D]

Answer 2

As I note in the comments, STRING_SPLIT has a big caveat in the documentation:
The order is not guaranteed to match the order of the substrings in the input string.
As a result, you're safer off using a function that gives you the ordinal position. In this case I use DelimitedSplit8K_LEAD and then assume you are using SQL Server 2017+:
DECLARE #YourString varchar(20) = 'A|B|C|D';
WITH Splits AS(
SELECT DS.ItemNumber,
DS.Item
FROM dbo.DelimitedSplit8K_LEAD(#YourString,'|') DS),
Groups AS(
SELECT S1.ItemNumber,
CONCAT('[',STRING_AGG(S2.Item,'|') WITHIN GROUP (ORDER BY S2.ItemNumber),']') AS Agg
FROM Splits S1
JOIN Splits S2 ON S1.ItemNumber >= S2.ItemNumber
GROUP BY S1.ItemNumber)
SELECT STRING_AGG(Agg,',') WITHIN GROUP (ORDER BY ItemNumber)
FROM Groups;
If you aren't on SQL Server 2017+, you'll need to use the "old" FOR XML PATH (and STUFF) method.
DB<>Fiddle

Answer 3

Just in case you don't want (or can't use) that SUPER DelimitedSplit8K_LEAD, here is an XML approach that will maintain the sequence
Example
Declare #S varchar(max) = 'A|B|C|D'
;with cte as (
Select RetSeq = row_number() over (order by 1/0)
,RetVal = ltrim(rtrim(B.i.value('(./text())[1]', 'varchar(max)')))
From (Select x = Cast('<x>' + replace((Select replace(#S,'|','§§Split§§') as [*] For XML Path('')),'§§Split§§','</x><x>')+'</x>' as xml).query('.')) as A
Cross Apply x.nodes('x') AS B(i)
), cte1 as (
Select *
,Comb ='['+stuff((select '|' +RetVal From cte Where RetSeq<=A.RetSeq Order By RetSeq For XML Path ('')),1,1,'') +']'
From cte A
Group By RetSeq,RetVal
)
Select NewValue = stuff((select ',' +Comb From cte1 Order By RetSeq For XML Path ('')),1,1,'')
Returns
NewValue
[A],[A|B],[A|B|C],[A|B|C|D]

Answer 4

Starting with v2016 there is JSON, which allows for a position-safe splitter using a JSON array. The path can be built with a recursive CTE:
DECLARE #yourString VARCHAR(MAX) ='A|B|C|D';
WITH cte AS
(
SELECT A.[key] AS ItmIndex
,A.[value] AS ItmVal
FROM OPENJSON(CONCAT('["',REPLACE(#yourString,'|','","'),'"]')) A
)
,rcte AS
(
SELECT ItmIndex, ItmVal
,CAST(ItmVal AS VARCHAR(MAX)) AS Result
FROM cte
WHERE ItmIndex=0
UNION ALL
SELECT cte.ItmIndex, cte.ItmVal
,CAST(CONCAT(rcte.Result,'|',cte.ItmVal) AS VARCHAR(MAX))
FROM cte
INNER JOIN rcte ON cte.ItmIndex=rcte.ItmIndex+1
)
SELECT * FROM rcte;
The idea in short:
The first cte will transform your string into a set with a guaranteed sort order (other than STRING_SPLIT()).
The second cte will start with the array's index 0 and then travers the list adding each item to the growing string.