8. Average customer spend by month easy

Instruction

• Write a query to return the average customer spend by month.
• Definition: average customer spend: total customer spend divided by the unique number of customers for that month.
• Use EXTRACT(YEAR from ts_field) and EXTRACT(MONTH from ts_field) to get year and month from a timestamp column.
• The order of your results doesn't matter.

Table: payment sample data

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Movie rental payment transactions table

   col_name   | col_type
--------------+--------------------------
 payment_id   | integer
 customer_id  | smallint
 staff_id     | smallint
 rental_id    | integer
 amount       | numeric
 payment_ts   | timestamp with time zone

Sample results

year	mon	avg_spend
2020	2	3.2543037974683544
2020	5	9.1301559454191033