Rolling 30-Day Retention Query | Dataford Interview Questions

Your question is Rolling 30-Day Retention Query. Start with the requirements and the two tables on the right.

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Problem

EvenUp's marketing team wants to compare how effectively acquisition channels drive early customer engagement. Using customer acquisition records and subsequent activity events, calculate 30-day retention for each acquisition-date and channel cohort.

Assume a customer is retained when they generate at least one activity event strictly after acquisition and before 30 days have elapsed. Include cohorts with zero retained customers, and label missing acquisition channels as Unknown.

Requirements

Return the acquisition date, acquisition channel, cohort size, retained customer count, and retention rate.
Count each customer at most once, even if they have multiple activity events.
Use a LEFT JOIN so cohorts with no retained customers remain in the output.
Exclude events exactly 30 days after acquisition.
Order results by acquisition channel and acquisition date.

Schema

customers

Column	Type	Description
customer_idPK	INTEGER	Unique customer identifier
acquired_at	TIMESTAMPTZ	Timestamp when the customer was acquired
acquisition_channel	VARCHAR(50)	Marketing channel credited with acquisition

customer_events

Column	Type	Description
event_idPK	INTEGER	Unique activity event identifier
customer_id	INTEGER	Customer associated with the event
event_at	TIMESTAMPTZ	Timestamp when the activity occurred
event_type	VARCHAR(50)	Type of customer activity