client_id <- (2260419,2260412,2260413,2260415)
  transaction_date <- (2016-09-03, 2016-09-04, 2016-09-06, 2016-09-07) 
  amount <- (350.0, 250.0,431.0,220.0)
month_ARPU AS
(SELECT
  visit_month,
  Avg(revenue) AS ARPU 
  FROM
  (SELECT
    Cust_id,
    Datediff(MONTH, ‘2010-01-01’, transaction_date) AS visit_month,
    Sum(transaction_size) AS revenue 
    FROM   transactions 
    WHERE  transaction_date > Dateadd(‘year’, -1, CURRENT_DATE)
    GROUP BY
    1,
    2)
  GROUP BY 1)
I understand how SELECT, GROUP BY, COUNT(1), SUM() and AS() works individually but not as a whole like in the code above, mainly how COUNT(1) and SUM() are working.
 
     
    