如何使用 jQuery、AJAX、PHP 和 MySQL 建立動態相關下拉式選單？

$().ready(function () {
    $("#item_1").change(function () {
        var group_id = $(this).val();

        $.ajax({
            type: "POST",
            url: "../../db/groups.php?item_1_id=" + group_id,
            dataType: "json",
            success: function (data) {
                // Clear previous options
                $('select#item_2').empty();
                $('select#item_2').append('<option value="0">Select Option</option>');

                // Populate options from JSON
                $.each(data.subjects, function (i, val) {
                    $('select#item_2').append('<option value="' + val.id + '">' + val.name + '</option>');
                });

                $('select#item_2').focus();
            },
            beforeSend: function () {
                $('select#item_2').empty();
                $('select#item_2').append('<option value="0">Loading...</option>');
            },
            error: function () {
                $('select#item_2').attr('disabled', true);
                $('select#item_2').empty();
                $('select#item_2').append('<option value="0">No Options</option>');
            }
        });
    });
});