WITH recursive input_strings AS (
    SELECT
        CAST(:input_string AS VARCHAR(1024)) AS mystring
    FROM rdb$database
), input_data AS (
    SELECT
        1 AS n,
        mystring AS all_data,
        (CHAR_LENGTH(mystring) - POSITION(',', reverse(mystring))) AS lim,
        SUBSTRING(mystring FROM 1 FOR (POSITION(',', mystring, 1) - 1)) AS column_value
    FROM input_strings
    UNION ALL
    SELECT
        POSITION(',', all_data, n) + 1,
        all_data,
        lim,
        SUBSTRING(all_data FROM
            (POSITION(',', all_data, n + 1) + 1) FOR (
                POSITION(',', all_data, n + 2) - POSITION(',', all_data, n + 1) - 1
            )
        )
    FROM input_data
    WHERE
        n < lim
)
SELECT CAST(column_value AS INT) AS column_value FROM input_data
UNION ALL
SELECT CAST(
    SUBSTRING(mystring FROM
        (CHAR_LENGTH(mystring) - POSITION(',', reverse(mystring))) + 2
    ) AS INT)
FROM input_strings