# Calculate Monthly Customer Retention Write one PostgreSQL SELECT statement or CTE query. Do not create, alter, or modify tables. ## Schema orders | column | type | description | |---|---|---| | order_id | integer | Unique order identifier | | customer_id | integer | Customer identifier | | order_ts | timestamp | Time the order was placed | | status | text | completed or cancelled | ## Task For every calendar month represented by at least one completed order, calculate customer retention from that month to the immediately following calendar month. A customer belongs to a month if they placed at least one completed order in it. Retention is the percentage of distinct customers in the cohort month who also placed a completed order in the next calendar month. Omit the final month because its following month is not observable. ## Required Output Return cohort_month as a date, cohort_customers, retained_customers, and retention_pct rounded to two decimal places. Sort by cohort_month ascending. ## Constraints - Count each customer at most once per month. - Cancelled orders do not establish activity. - A missing calendar month means retention to that month is zero, not retention to the next observed month. ```hint Build customer-month activity Deduplicate completed orders to one row per customer and calendar month, then self-join on exactly one month later. ```