title: check if the username exist in Firebase
body: <p>I have a database like this</p>
  
  <p>users/UID1/"Username" (of user 1)</p>
  
  <pre><code> /UID2/"Username" (of user 2)
   /UID3/"Username" (of user 3)
   /UID4/"Username" (of user 4)
  </code></pre>
  
  <p>and so on..</p>
  
  <p>I would like to check if the username exist but I don't manage to go in a loop with all existing UID.
  For now I have tried :</p>
  
  <pre><code>let databaseRef = Database.database().reference()        
              databaseRef.child("users").child("uid").child("Username").observeSingleEvent(of: DataEventType.value, with: { (snapshot) in
                  switch snapshot.value {
                  case let value as Bool where value:
                      // value was a Bool and equal to true
                      print ("username found")
                  default:
                      // value was either null, false or could not be cast
                      print ("username not found")
                  }
              })
  
          }
  </code></pre>
  
  <p>I don't know what to put instead of child("uid") to loop into every uid in my database and check if a username exists</p>
  
  <p>Thanks for your help !</p>