Select data from 2 MySQL tables - PHP

I have two tables:

Table 1

• contacts
  • id
  • email
  • name
  • lastname
  • postcode
  • straat
  • huisnr
  • woonplaats
  • klantnummer
  • bsn
  • land
  • debiteurnummer

Table 2

• contacts_group
  • id
  • mail
  • group_name

How can I select and order this 2 tables in one query. I've tried union and left join, but did not work.

$result = mysqli_query($database->connection,
    "SELECT * 
     FROM contacts 
     WHERE owner = '$session->username'
     ORDER BY name ASC ,bedrijfsnaam ASC") 
or die(mysqli_error());

while($roww = mysqli_fetch_array($result)){
    echo $roww['email'];
    echo $roww['name']; 
}

Table contacts_group:

$result = mysqli_query($database->connection,
    "SELECT * 
     FROM contacts_group 
     WHERE owner = '$session->username'
     ORDER BY group_name ASC") 
or die(mysqli_error());

while($roww = mysqli_fetch_array($result)){
   echo $roww['mail'];
   echo $roww['group_name']; 
}