SQL for Channel Sign-Ups and Growth | Dataford Interview Questions

Turing's Product Growth team needs a weekly view of candidate acquisition performance. Write a PostgreSQL query that reports sign-ups and application conversion by acquisition channel.

A candidate is considered converted when they submit at least one Turing application within seven days after signing up. Include acquisition channels with sign-ups even when no candidate converted.

Requirements

Group sign-ups by the Monday of the signup week and acquisition_channel.
Calculate weekly sign-ups, converted candidates, and conversion rate as a percentage.
Calculate week-over-week sign-up growth for the same acquisition channel. Return NULL when no prior-week comparison exists.
Include the supplied reporting period from January 1 through January 31, 2025, and sort by week followed by acquisition channel.

Schema

users

Column	Type	Description
user_idPK	INT	Unique Turing candidate identifier
signup_at	TIMESTAMP	Timestamp when the candidate created an account
acquisition_channel	VARCHAR(50)	Marketing acquisition source

applications

Column	Type	Description
application_idPK	INT	Unique application identifier
user_id	INT	Candidate who submitted the application
submitted_at	TIMESTAMP	Application submission timestamp

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Problem