我的mySQL查询有什么问题?左连接/内连接

基本上，这个脚本将我们小系统中的所有订单导出为CSV文件。有4个独立的表被连接以完成此操作。SEorder, SEtracking, SE_user, users_orders。

错误如下:

<div id="Error">An error occurred in script '/home/sites/mywebsite.com/web/administration/allorders.php' on line 81:
<br />Undefined index:  UserLast

    //Make the query-select all orders
$query2 ="SELECT DISTINCT o.OrderID, Basket, Title, FirstName, LastName, Suffix, Company, Address1, Address2, City, State, Zip, GiftCard, GiftCardMsg, t.Tracking, u.UserFirst, u.UserLast AS doo ".
"FROM SEorder o ".
"LEFT JOIN SEtracking t ON (o.OrderID = t.OrderID) ".
"INNER JOIN users_orders uo ON (uo.OrderID = o.OrderID) ".
"INNER JOIN SEuser u ON (u.UserID = uo.UserID) ".
"AND Submitted='Y' ".
"ORDER BY OrderDate ASC";

，这里是我的PHP获取数据。这是抛出错误的第81行:

$username = $row['UserFirst'] . " " . $row['UserLast'];

我确信表SEuser存在，有列UserLast，并且该列中有数据。我不是完全精通JOIN，所以我在这里错过了什么?我没有创建这个脚本，只是排除故障。谢谢!