获取 SQL 记录

当我运行此代码时，它返回的主题很好...

$query = mysql_query("SELECT topic
FROM question 
WHERE id = '$id'");
    if(mysql_num_rows($query) > 0) {
        $row = mysql_fetch_array($query) or die(mysql_error());
        $topic = $row['topic'];
    }

但是当我将其更改为此时，它根本无法运行。 为什么会这样？

$query = mysql_query("SELECT topic, lock
FROM question 
WHERE id = '$id'");
    if(mysql_num_rows($query) > 0) {
        $row = mysql_fetch_array($query) or die(mysql_error());
        $topic = $row['topic'];
        $lockedThread = $row['lock'];
        echo "here: " . $lockedThread;
    }