L7 Power-User Curve

SQLHardSenior level~15 min

Problem

**[Classic — product analytics]**

The L7 power-user curve shows how many users were active for exactly N days
in a 7-day window.

For the window **2023-01-01 to 2023-01-07**, return a histogram of users
by number of distinct active days.

Return `days_active` and `user_count`, ordered by days_active.

Example rows — the live problem includes the full dataset.

activity

activity_id	user_id	activity_date

Your answer should return 3 rows with the columns days_active, user_count.

SELECT *
FROM activity;

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