UNION查询的问题

我正在尝试什么似乎是一个简单的联合查询在我的数据库中的两个表，虽然我无法检索成功的数组结果。应该注意的是，每个表都有不同的列，并且我想创建一个虚拟结果(promo AS origin[用于从'venpromo'表中提取的结果])&&作为origin[从'vacation'表中提取的结果])，以便将结果排序到不同的数组结构中。我到处都看了，union都使用不同的语法。提前感谢大家!

<?php
    require_once('includes/config.php');
    $event_query = "SELECT *, promo AS origin FROM venpromo
    UNION 
    SELECT *, vacation AS origin FROM venpromo 
    ORDER BY popularity DESC";
    $event_result = mysql_query($event_query, $connection);
    while ($event = mysql_fetch_array($event_result)) {
        if ($event['origin'] == "promo") {
            $event_array[] = array(
                'id' => $event['id'],
                'title' => $event['calname'],
                'start' => $event['sdate'],
                'end' => $event['edate'],
                'color' => "red",
                'url' => "http://www.norrisportal.com/bulletinpost.php?id=" . $event['id'] . "&hashkey=akdULjsjyUpYyTzOT7"
            );
        } elseif ($event['origin'] == "vacation") {
            $event_array[] = array(
                'id' => $event['id'],
                'title' => $event['reason'],
                'start' => $event['vacstart'],
                'end' => $event['vacend'],
                'color' => "blue"
            );
        }
    }
    echo json_encode($event_array);
?>

当访问页面查看结果时，我看到'null'。