SQL Server find chains between two columns - sql-server

Question

I have a table like this:
from | to
-----+-----
23 | 24
24 | 25
25 | 27
27 | 30
45 | 46
46 | 47
50 | 52
53 | 60
I need a SQL Server query that detect chain's and return min (from) and max (to) in each chain (also chain's with one record):
from | to
-----+-----
23 | 30
45 | 47
50 | 52
53 | 60

Answer 1

Here's an approach using a recursive CTE.
CREATE TABLE #chainLinks(linkFrom INTEGER, linkTo INTEGER);
INSERT INTO #chainLinks VALUES (23,24);
INSERT INTO #chainLinks VALUES (24,25);
INSERT INTO #chainLinks VALUES (25,27);
INSERT INTO #chainLinks VALUES (27,30);
INSERT INTO #chainLinks VALUES (45,46);
INSERT INTO #chainLinks VALUES (46,47);
INSERT INTO #chainLinks VALUES (50,52);
INSERT INTO #chainLinks VALUES (53,60);
WITH reccte AS
(
/*Recursive Seed*/
SELECT linkFrom AS chainStart,
linkFrom,
linkTo,
0 as links
FROM #chainLinks as chainLinks
WHERE linkFrom NOT IN (SELECT DISTINCT linkTo FROM #chainLinks)
UNION ALL
/*Recursive Term*/
SELECT
reccte.chainStart,
chainLinks.linkFrom,
chainLinks.linkTo,
links + 1
FROM reccte
INNER JOIN #chainLinks as chainLinks ON reccte.linkTo = chainLinks.linkFrom
)
SELECT chainStart, linkTo AS chainEnd
FROM
(
SELECT chainStart, linkFrom, linkTo, links, ROW_NUMBER() OVER (PARTITION BY chainStart ORDER BY links DESC) AS rn
FROM reccte
)subrn
WHERE rn = 1;
A recursive CTE takes two parts
A recursive seed - This is the part above the UNION where we determine which records from our table begin the recursion. Here we want any linkFrom that isn't also a linkTo
A recusrive term - This is the part below the UNION where we join the cte called reccte back to the original table. This part of the CTE iterates over and over again until that join fails.
In here we are also tracking that links which is just a counter of the number of iterations we have gone through to get to that outputted record. We keep the highest number of links for each starting point chainStart.
Here is the working example: https://rextester.com/JWUW57837

Answer 2

If there are branches within the chains it become a little bit more tricky.
In the sample data below, there's a split on From=12.
So the result shows 2 chains starting from 14.
create table yourtable (
[From] int not null,
[To] int not null,
PRIMARY KEY ([From],[To])
)
GO
✓
insert into yourtable
([From],[To]) values
(2,3),(3,5),(5,4)
,(14,12),(12,15),(15,11),(11,10)
,(12,9)
,(21,23)
GO
9 rows affected
;WITH RCTE_CHAINS AS
(
-- seeding with the start of chains
SELECT [From] AS MinFrom, [From], [To], 0 AS Lvl
, CAST(IIF(EXISTS(
SELECT 1 FROM YourTable n
WHERE n.[From] = t.[To]
),1,0) AS BIT) AS hasNext
FROM YourTable t
WHERE NOT EXISTS
(
SELECT 1
FROM YourTable t2
WHERE t2.[To] = t.[From]
)
UNION ALL
-- looping through the childs
SELECT c.MinFrom, t.[From], t.[To], c.Lvl+1
, CAST(IIF(EXISTS(
SELECT 1 FROM YourTable n
WHERE n.[From] = t.[To]
),1,0) AS BIT) AS hasNext
FROM RCTE_CHAINS c
JOIN YourTable t ON t.[From] = c.[To]
)
SELECT MinFrom AS [From], [To]
FROM RCTE_CHAINS
WHERE hasNext = 0
GO
From | To
---: | -:
21 | 23
14 | 9
14 | 10
2 | 4
db<>fiddle here