Get The Topmost Parent's ID, Regardless of The Child's ID

I have a table (eg the name is "page"), where each row is a child of another row, like below (see the HTML section):

I want to find the topmost parent's id, by using a child's id. For example, we get a URL variable as the id of the bottommost child:

Then we store the URL variable into a variable:

$p = $_GET['p'];

To get the topmost parent's id, I have to use query function several times. For example:

if($result = $mysqli->query("SELECT * FROM page WHERE id='$p' LIMIT 1"))
    while($row = $result->fetch_array())
        $parent = $row["parent"];

if($result = $mysqli->query("SELECT * FROM page WHERE id='$parent' LIMIT 1"))
    while($row = $result->fetch_array())
        $parent1 = $row["parent"];

if($result = $mysqli->query("SELECT * FROM page WHERE id='$parent1' LIMIT 1"))
    while($row = $result->fetch_array())
        $parent2 = $row["parent"];

if($result = $mysqli->query("SELECT * FROM page WHERE id='$parent2' LIMIT 1"))
    while($row = $result->fetch_array())
        $id = $row["id"];

This way is ineffective, because if I got another URL variable as the id of another child of the higher/ lower level, this will not work.

How to get the topmost parent's id dynamically, regardless of the child's id?



CREATE FUNCTION `fnGetRootParent`(
    page INT
) RETURNS int(11)
    DECLARE _parent INT;
    DECLARE _result INT;

    SET _parent = page;

    my_loop: LOOP
        FROM page p
        WHERE id = _parent;

        IF (_parent = 0) OR (_parent = _result) THEN
            LEAVE my_loop;
        END IF;
    END LOOP my_loop;

    RETURN _result;

SELECT fnGetRootParent(213);

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