Oracle 11g Release 2

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WITH yourTable AS (
    SELECT 1 AS question_id, '7' AS element_id FROM dual UNION ALL
    SELECT 1, '8' FROM dual UNION ALL
    SELECT 1, 'z' FROM dual UNION ALL
    SELECT 2, 'x' FROM dual UNION ALL
    SELECT 2, '9' FROM dual UNION ALL
    SELECT 2, '10' FROM dual UNION ALL
    SELECT 3, '10' FROM dual UNION ALL
    SELECT 3, '11' FROM dual UNION ALL
    SELECT 3, '12' FROM dual UNION ALL
    SELECT 3, 'y' FROM dual
),
cte AS (
    SELECT t.*, ROW_NUMBER() OVER (PARTITION BY question_id
                                   ORDER BY element_id) rn
    FROM yourTable t
),
cte2 AS (
    SELECT
        question_id,
        element_id,
        CASE WHEN rn = 1
             THEN MAX(CASE WHEN REGEXP_LIKE(element_id, '^[A-Za-z]+$')
                           THEN element_id END)
                  OVER (PARTITION BY question_id) END AS element
    FROM cte
)

SELECT
    question_id,
    element_id,
    element
FROM cte2
WHERE
    REGEXP_LIKE(element_id, '^[0-9]+$')
ORDER BY
    question_id,
    element_id;

QUESTION_ID	ELEMENT_ID	ELEMENT
1	7	z
1	8	null
2	10	x
2	9	null
3	10	y
3	11	null
3	12	null