Complete Churn Rate calculation for multiple months

WITH months AS (
  SELECT 
    '2017-01-01' AS first_day, 
    '2017-01-31' AS last_day 
  UNION 
  SELECT 
    '2017-02-01' AS first_day, 
    '2017-02-28' AS last_day 
  UNION 
  SELECT 
    '2017-03-01' AS first_day, 
    '2017-03-31' AS last_day
), 
cross_join AS (
  SELECT *
  FROM subscriptions
  CROSS JOIN months
), 
status AS (
  SELECT 
    id, 
    first_day AS month, 
    CASE
      WHEN (subscription_start < first_day) 
        AND (
          subscription_end > first_day 
          OR subscription_end IS NULL
        ) THEN 1
      ELSE 0
    END AS is_active, 
    CASE
      WHEN subscription_end BETWEEN first_day AND last_day THEN 1
      ELSE 0
    END AS is_canceled 
  FROM cross_join
), 
status_aggregate AS (
  SELECT 
    month, 
    SUM(is_active) AS active, 
    SUM(is_canceled) AS canceled 
  FROM status 
  GROUP BY month
) 
SELECT
  month, 
  1.0 * canceled / active AS churn_rate 
FROM status_aggregate;

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