There's a users table with the columns user_id and signup_date, and an events table with the columns user_id and event_time. Write a SQL query that, for each weekly signup cohort, returns the cohort size and day-7 retention: the share of users who had at least one event exactly seven days after signing up.

How to approach it

The interviewer wants to see whether you can compute a product metric correctly in SQL. It's important not to lose users without events from the denominator, not to multiply rows with a join, not to get zero from integer division, and not to distort fresh cohorts. Just as important is clarifying the definition of the metric first, and only then writing code.