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73. Number of days to become a happy customer medium

Instructions:

  • Any customers who made 10 movie rentals are happy customers
  • Write a query to return the average number of days for a customer to make his/her 10th rental.
  • If a customer has never become a ‘happy’ customer, you should skip this customer when computing the average.
  • You can use EXTRACT(DAYS FROM tenth_rental_ts - first_rental_ts) to get the number of days in between the 1st rental and 10th rental
  • Use ROUND(average_days) to return an integer

Table: rental

   col_name   | col_type
--------------+--------------------------
 rental_id    | integer
 rental_ts    | timestamp with time zone
 inventory_id | integer
 customer_id  | smallint
 return_ts    | timestamp with time zone
 staff_id     | smallint
 last_update  | timestamp with time zone

  

Sample results

 avg
-------
    65

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