Aggregate User Events and Check Join Uniqueness
Company: Mixpanel
Role: Data Scientist
Category: Data Manipulation (SQL/Python)
Difficulty: medium
Interview Round: Technical Screen
## Build a User Activity Summary from Event Data
You have two tables. The intended user summary contains one row per `user_id`, and the `users` table provides one record for each user.
| Table | Column | Meaning |
|---|---|---|
| `users` | `user_id` | User identifier. |
| `users` | `signup_date` | The user's signup date or timestamp. |
| `users` | `plan_type` | The user's plan type. |
| `events` | `user_id` | Identifier of the user associated with an event. |
| `events` | `event_ts` | Timestamp of the event. |
Write one PostgreSQL query that returns every user together with that user's event summary. Group all available event rows by `user_id` and compute:
- `event_cnt`: the number of non-NULL `event_ts` values.
- `active_days`: the number of distinct calendar dates represented by non-NULL `event_ts` values. Use the date component of the stored timestamp.
- `first_event`: the earliest non-NULL `event_ts` value.
Join the event summary to `users` so that users without any event rows are retained. For such users, the three event-summary fields are NULL; do not replace them with zero. Do not add a first-week restriction or any other time filter to the aggregation.
Return these columns in this order:
`user_id`, `signup_date`, `plan_type`, `event_cnt`, `active_days`, `first_event`.
The output row order is unrestricted. The query must not change either input table.
### Clarifications
The event summary has one row per `user_id`. The final result must retain the intended one-row-per-user invariant; the practical exercise also asks you to check that merging the summaries has not introduced duplicate user identifiers.
Overview: Aggregate user events and retain all users in PostgreSQL, computing event count, distinct active dates, earliest event, and join uniqueness.
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