no database selected

$conn=mysql_connect("localhost","","");
		if($conn)
		{
			$db=mysql_select_db("kart",$conn);
			$query="select * from category";
			$result=mysql_query($query,$conn);
			if($result)
			{
				$row=mysql_num_rows($result);
				echo($row);
			}
			else
			{
				echo(mysql_error($conn));
			}
		}
		else
		{
			echo("hello");
		}