MySQLI根据以前的选择和表格显示下拉选项

I have two tables - companies and clients. They each have an identical row labeled company_id so I can assign a company to each client.

First a company is created.

Then the client is created and the company is associated with it in the process.

Then an order is created...

For orders, I have a table named orders. When inserting data into the table, I have a form with two dropdown boxes. The first dropdown is to select the company the order is for. This is populated using the following:

$getCompany = mysqli_query($db, "SELECT * FROM companies ORDER BY company_name ASC");

<label for="company_id">Company</label>
<select class="form-control" name="company_id" id="company_id">
    <option disabled selected></option>
    <?php

        // If there are results, output each row.
        if($getCompany) {
            while($company = mysqli_fetch_assoc($getCompany)) { ?>
                <option value="<?php echo $company['company_id']; ?>">
                    <?php echo $company['company_name']; ?>
                </option>
            <?php }
        } 
    ?>
</select>

The second dropdown is identical, but instead calls the clients' names.

$getClient  = mysqli_query($db, "SELECT * FROM clients ORDER BY client_fname ASC");

<select class="form-control" name="client_id" id="client_id">
    <option disabled selected></option>
    <?php

        // If there are results, output each row.
        if($getClient) {
            while($client = mysqli_fetch_assoc($getClient)) { ?>
                <option value="<?php echo $client['client_id']; ?>">
                    <?php echo $client['client_fname']." ".$client['client_lname']; ?>
                </option>
            <?php }
        } 
    ?>
</select>

If I select CompanyA from the first dropdown, how can I show only the clients associated with that selection in the second dropdown?

Use ajax like this

Javascript

 $('.company_id').change(function()
   {
       var company_id = $('.company_id').val();
        var request = $.ajax({
                url: "your_ajax.php",
                method: "POST",
                data: {
                    company_id: company_id,
                    action: 'get_clients_by_company_id'

                }
            });
            request.done(function (msg) {
                $('#client_id').val(msg);
            });

   });

PHP CODE IN YOUR AJAX

 <?php
if (isset($_POST['action']) and $_POST['action'] == 'get_clients_by_company_id') {
    $company_id = $_POST['company_id'];
    if (!empty($company_id)) {
        $getClient = mysqli_query($db, "SELECT * FROM clients WHERE company_id = '$company_id' ORDER BY client_fname ASC");
        while ($client = mysqli_fetch_assoc($getClient)) {
            ?>
            <option value="<?=$client['client_id']; ?>">
                <?=$client['client_fname'] . " " . $client['client_lname']; ?>
            </option>

            <?php
        }
    }
}
?>