Rolling 30-Day Subscriber Usage Totals | Dataford Interview Questions

Business Context

Bell Mobility analytics needs a daily view of subscriber data consumption to support network capacity monitoring and usage-based reporting.

Write a PostgreSQL query that calculates each active Bell Mobility subscriber's rolling 30-day data usage from the recorded usage events.

Join usage events to active subscribers only.
Aggregate multiple usage events for the same subscriber and date into daily_usage_mb.
Use a window function partitioned by subscriber to calculate rolling_30_day_mb. The window must include the current date and the preceding 29 calendar days.
Treat NULL usage values as zero and order the final output by subscriber and usage date.

bell_subscribers

Column	Type	Description
subscriber_idPK	INT	Unique Bell subscriber identifier
subscriber_name	VARCHAR(100)	Subscriber display name
status	VARCHAR(20)	Current subscriber account status

bell_data_usage

Column	Type	Description
usage_event_idPK	INT	Unique data usage event identifier
subscriber_id	INT	Subscriber associated with the usage event
usage_date	DATE	Date on which usage was recorded
usage_mb	NUMERIC(10,2)	Data consumed in megabytes

Your question is Rolling 30-Day Subscriber Usage Totals. Start with the requirements and the two tables in the Question tab.

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