SQL Server 2019

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CREATE TABLE Table1
    ([Child] int, [Parent] int)
;
    
INSERT INTO Table1
    ([Child], [Parent])
VALUES
    (1, 2),
    (1, 3),
    (2, 3)
;

3 rows affected

INSERT INTO table1 (child, parent)
SELECT DISTINCT t1.parent, NULL
FROM table1 t1
LEFT JOIN table1 t2
    ON t1.parent = t2.child
WHERE t2.child IS NULL;

1 rows affected

SELECT *
FROM Table1


Child
Parent


1
2

1
3

2
3

3
null