SQL Server 2017

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with t as (
      select *
      from (values (1, 2), (2, 7), (5, 7), (7, 10), (10, 11), (12, 13), (13, 14)) v(x, y)
     ),
     tt as (
      select v.x, v.y
      from t cross apply
           (values (x, y), (y, x)) v(x, y)
     ),
     cte as (
      select (case when tt.x < tt.y then tt.x else tt.y end) as lowest, v.val, tt.x, tt.y, convert(varchar(max), concat(',', tt.x, ',', tt.y, ',')) as vals
      from tt cross apply
           (values (x), (y)) v(val)
      union all
      select (case when tt.y < cte.lowest then tt.y else cte.lowest end) as lowest, cte.val, cte.x, tt.y, concat(cte.vals, tt.y, ',') as vals
      from cte join
           tt
           on cte.y = tt.x and cte.vals not like concat('%,', tt.y, ',%')
      union all
      select (case when tt.x < cte.lowest then tt.x else cte.lowest end) as lowest, cte.val, tt.x, cte.y, concat(cte.vals, tt.x, ',') as vals
      from cte join
           tt
           on cte.x = tt.y and cte.vals not like concat('%,', tt.x, ',%')
     )
select min(lowest), val
from cte
group by val

(No column name)	val
1	1
1	2
1	5
1	7
1	10
1	11
12	12
12	13
12	14