How to Tackle Month-over-Month Churn Rate in SQL Interview Questions

Churn rate questions are a staple in data engineering and business intelligence interviews, testing candidates on time-based logic and subscription data handling. The challenge involves a subscriptions table where a NULL end_date indicates an active subscription, and the goal is to calculate monthly churn rates throughout 2025. The approach uses a common table expression to define each month alongside its previous and next periods, then joins subscription records to identify which users were active in any given month. Churned users are identified as those active in the prior month but absent in the current one, using a LEFT JOIN and NULL check pattern. The final churn rate is computed by dividing churned users by the previous month's active user count, multiplied by 100 to return a percentage.