判断空结果(PHP、PDO 和 MySQL)

我做错了什么？我只是从表中检索结果，然后将它们添加到数组中.一切都按预期进行，直到我判断结果为空...

这将获取匹配项，将其添加到我的数组中，并按预期 echo 结果:

$today = date('Y-m-d', strtotime('now'));

$sth = $db->prepare("SELECT id_email FROM db WHERE hardcopy = '1' AND hardcopy_date <= :today AND hardcopy_sent = '0' ORDER BY id_email ASC");

$sth->bindParam(':today', $today, PDO::PARAM_STR);

if(!$sth->execute()) {
    $db = null;
    exit();
}

while ($row = $sth->fetch(PDO::FETCH_ASSOC)) {
    $this->id_email[] = $row['id_email'];
    echo $row['id_email'];
}

$db = null;
return true;

当我try 判断空结果时，我的代码返回"empty"，但不再产生匹配结果:

$today = date('Y-m-d', strtotime('now'));

$sth = $db->prepare("SELECT id_email FROM db WHERE hardcopy = '1' AND hardcopy_date <= :today AND hardcopy_sent = '0' ORDER BY id_email ASC");

$sth->bindParam(':today',$today, PDO::PARAM_STR);

if(!$sth->execute()) {
    $db = null;
    exit();
}

if ($sth->fetchColumn()) {
    echo 'not empty';
    while ($row = $sth->fetch(PDO::FETCH_ASSOC)) {
        $this->id_email[] = $row['id_email'];
        echo $row['id_email'];
    }
    $db = null;
    return true;
}
echo 'empty';
$db = null;
return false;