SQLite 3.27

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CREATE TABLE Tree
       (parent INTEGER,
        child  INTEGER);
INSERT INTO Tree
       VALUES -- (parent -> child)
              (1, 2), (1, 3), (1, 4),
              (2, 5), (2, 11), (3, 9),
              (5, 6), (5, 7),  (5, 8),
              (9, 10), (11, 12);
WITH RECURSIVE children_i
               (parent,
                child)
AS (
-- anchor/initial values
SELECT t1.parent,
       t1.child
       FROM tree t1
       WHERE t1.parent = 2
             AND NOT EXISTS (SELECT *
                                    FROM tree t2
                                    WHERE t2.parent = t1.parent
                                          AND t2.child < t1.child)
UNION ALL
-- recursion
SELECT c1.child,
       t1.child
       FROM tree t1
            INNER JOIN children_i c1
                       ON t1.parent = c1.child)
SELECT *
       FROM children_i;


parent
child


2
5

5
6

5
7

5
8