7. Unique customers count by month easy

Instruction

• Write a query to return the total number of unique customers for each 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: rental sample data

---

   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

Sample results

 year | mon | uu_cnt
------+-----+--------
 2020 |   1 |    123
 2020 |   2 |    456
 2020 |   3 |    789