特定列php/mysql的结果为空

当在数据库上执行时，我的查询返回正确的数据，但当实时运行时，company_title返回null。所有其他领域都有效。$row['company_name]是我无法使用的代码：

$query = "select * from invoices i, company_lookup cl, students s where i.company_id = cl.company_id and i.student_id = s.student_id;";
$results = $DB->query($query);
$invoices = mysql_query("select * from invoices");
?>
<table border="1" id="hl" name="hl">
    <tr>
    <th>Month/Year</th>
    <th>Full Amount</th>
    <th>Company</th>
</tr>
<?php while ($row = mysql_fetch_array($invoices)) { 
    $invoice_date = $row['invoice_date'];
    ?>
    <tr onMouseOver="showInvoicePayments(<? echo $row['invoice_id'] ?>);this.bgColor = '#C0C0C0'" onMouseOut ="this.bgColor = '#FFFFFF'" bgcolor="#FFFFFF"> 
        <td><? echo  date('F Y',strtotime($invoice_date)) ?></td>
        <td><? echo '$' . $row['full_amount'] ?></td>
        <td><? echo $row['company_name'] ?></td>
    </tr>
<?
}
?>