Duvidas com msql_query

Gostaria de saber oque estou fazendo de errado.

ps: sou bem inexperiente, ja li algumas coisas mas nao cntendi muito bem

class Sql{

	public $host, $user, $pass, $query;

	public function __construct(){
 mysqli_connect($this -> host = 'localhost', $this -> user = 'root', $this -> pass = '' );
 mysqli_select_db('cadastro');
	}
	public function query($qr){
 $this -> query = mysqli_query($qr);
 return $this -> query;
	}
	public function retornar(){
 return $this -> query;
}
	}
//$sql = new sql;
$nome = $_POST{'nome'};
$login = $_POST{'login'};
$pwd = $_POST{'senha'};
$text = $_POST;
 if(empty($nome)){
 echo "<script>alert('Preencha todos os campos para se cadastrar.'); history.back();</script>";
	}elseif(empty($login)){
 echo "<script>alert('Preencha todos os campos para se cadastrar.'); history.back();</script>";
	}elseif(empty($pwd)){
 echo "<script>alert('Preencha todos os campos para se cadastrar.'); history.back();</script>";
	}else{
 $query1 = mysqli_num_rows(mysqli_query("SELECT * FROM USUARIO WHERE LOGIN = '$login'"));
 if($query1 == 1){
 echo "<script>alert('Usuário já existe.'); history.back();</script>";
 }else{
 mysqli_query("insert into usuario (nome, login, senha,'NULL') values ('$nome','$login','$pwd','$text')");
 echo "<script>alert('Usuário cadastrado com sucesso.');</script>";
 //echo "<script>location.href='pagina-do-cliente-login.php';</script>";
 }
	}

?>

Warning: mysqli_query() expects at least 2 parameters, 1 given in C:\Program Files (x86)\EasyPHP-Devserver-16.1\eds-www\phpteste1\cadastro.php on line 30

Warning: mysqli_num_rows() expects parameter 1 to be mysqli_result, null given in C:\Program Files (x86)\EasyPHP-Devserver-16.1\eds-www\phpteste1\cadastro.php on line 30

Notice: Array to string conversion in C:\Program Files (x86)\EasyPHP-Devserver-16.1\eds-www\phpteste1\cadastro.php on line 34

Warning: mysqli_query() expects at least 2 parameters, 1 given in C:\Program Files (x86)\EasyPHP-Devserver-16.1\eds-www\phpteste1\cadastro.php on line 34