Array
(
    [SELECT STRAIGHT_JOIN d.parent
FROM `data` AS d] => Array
        (
            [0] => Array
                (
                    [parent] => 0
                )

            [1] => Array
                (
                    [parent] => 0
                )

            [2] => Array
                (
                    [parent] => 0
                )

            [3] => Array
                (
                    [parent] => 0
                )

            [4] => Array
                (
                    [parent] => 0
                )

            [5] => Array
                (
                    [parent] => 0
                )

            [6] => Array
                (
                    [parent] => 0
                )

            [7] => Array
                (
                    [parent] => 1
                )

            [8] => Array
                (
                    [parent] => 3
                )

            [9] => Array
                (
                    [parent] => 5
                )

            [10] => Array
                (
                    [parent] => 7
                )

            [11] => Array
                (
                    [parent] => 9
                )

            [12] => Array
                (
                    [parent] => 11
                )

            [13] => Array
                (
                    [parent] => 13
                )

        )

)