Monthly Upgrades and MoM Growth | Dataford Interview Questions

Write a query to find the monthly active users who upgraded their subscription tier, showing the month-over-month growth rate.

Use user_activity records to identify active users and subscription_changes records to identify upgrades occurring in the same calendar month. Include only users active during the month of their upgrade.

Output

One row per month with at least one qualifying user.
Columns: month, monthly_active_upgraded_users, and mom_growth_rate_pct.
Show growth as a percentage rounded to two decimal places. The first month should have a NULL growth rate.
Sort chronologically by month.

Schema

user_activity

Column	Type	Description
activity_idPK	INT	Unique activity record identifier
user_id	INT	HubSpot user identifier associated with the activity
activity_at	TIMESTAMP	Timestamp when the user was active

subscription_changes

Column	Type	Description
change_idPK	INT	Unique subscription change identifier
user_id	INT	HubSpot user identifier affected by the change
changed_at	TIMESTAMP	Timestamp when the subscription changed
change_type	VARCHAR(20)	Type of subscription change
old_tier	VARCHAR(30)	Subscription tier before the change
new_tier	VARCHAR(30)	Subscription tier after the change

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Problem