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

   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

Expected results

Solution postgres

SELECT 
	EXTRACT(YEAR FROM rental_ts) AS year,
	EXTRACT(MONTH FROM rental_ts) AS mon,
	COUNT(DISTINCT customer_id) AS uu_cnt
FROM rental
GROUP BY year, mon;
    

Explanation

This query is selecting data from a table called "rental" and extracting the year and month from a column called "rental_ts". It is also counting the number of distinct customer IDs and giving that count an alias "uu_cnt". The results are then grouped by year and month.

In summary, the query is counting the number of unique customers who rented something each month and displaying that count by year and month.



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