SQL Server 2017

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create table yourtable (
 [From] int not null, 
 [To] int not null,
 PRIMARY KEY ([From],[To])
)
insert into yourtable 
([From],[To]) values
(2,3),(3,5),(5,4)
,(14,12),(12,15),(15,11),(11,10)
,(12,9)
,(21,23)
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


From
To


21
23

14
9

14
10

2
4