public function countComment($diary_id)
{
if (empty(self::$conn)) {
self::$conn = $this->connectPdo();
}
$sql = "SELECT COUNT(content) FROM comment WHERE diary_id=?";
$stmt = self::$conn->prepare($sql);
$stmt->bindParam(1, $diary_id);
$dem = $stmt->execute();
$dem = $stmt->fetchColumn();
return $dem;
}