Wie gruppieren und pendeln Sie Tabellen mit Julia Dataframes?

Sagen wir, ich habe Dataframe

using DataFrames

df =DataFrame(Location = [ "NY", "SF", "NY", "NY", "SF", "SF", "TX", "TX", "TX", "DC"],
                 Class = ["H","L","H","L","L","H", "H","L","L","M"],
                 Address = ["12 Silver","10 Fak","12 Silver","1 North","10 Fak","2 Fake", "1 Red","1 Dog","2 Fake","1 White"],
                 Score = ["4","5","3","2","1","5","4","3","2","1"])

und ich möchte Folgendes tun:

1) eine Pivot-Tabelle mitLocation undClass was sollte @ ausgeb

Class     H  L  M
Location         
DC        0  0  1
NY        2  1  0
SF        1  2  0
TX        1  2  0

2) gruppiere nach "Ort" und zähle die Anzahl der Datensätze in dieser Gruppe, die @ ausgeben soll

   Pop  
DC  1   
NY  3  
SF  3  
TX  3