# Past Meets Present Meets Shared

THE CLOUD EXPEDITION! · level 17 · chapter 6

Chronos and Marina lock eyes. 'My time travel...' he begins. '...plus my shared ecosystem data...' she finishes. 'ONE QUERY.' They turn to you in unison. This is the Avengers moment — two paradigms, combined for the first time. Combine <span class='feature-highlight'>Time Travel + Shared Data</span> — joining historical snapshots with live shared databases.

## Task

Correlate storm damage with ecosystem readings. First compute the population change per district (version 1 vs current), then match each district to its ecosystem readings. Each district has several readings, so average them: return <b>district</b>, <b>pop_change</b>, <b>avg_temperature</b>, and <b>avg_water_quality</b> — biggest absolute population changes first. Tables: <code>dbquacks_living_lake.pond_census</code> (Ducklake, version 1 = pre-storm), <code>dbquacks_marina_lake.ecosystem_readings</code> (shared, has <code>pond_name</code>, <code>temperature</code>, <code>water_quality</code> — district names match pond_name).<br><br><b>Re-attach if needed:</b><ul><li><code>dbquacks_marina_lake</code> → <code>md:_share/marina_lake_db/3be07852-dfce-4b29-b415-99c9f7fdcc6a</code></li></ul>

## Hint

Combine time travel with shared data — district names match pond_name; average the readings per district with GROUP BY

## Expected result

Rows with columns: district, pop_change, avg_temperature, and avg_water_quality — one row per district, showing how storm population shifts correlate with ecosystem readings

## Tables

### `lake_visits` (500 rows)

| column | type |
| --- | --- |
| visit_id | BIGINT |
| duck_id | BIGINT |
| destination_pond | VARCHAR |
| visit_date | DATE |
| purpose | VARCHAR |

### `pondville_events` (50 rows)

| column | type |
| --- | --- |
| event_id | BIGINT |
| event_name | VARCHAR |
| event_date | DATE |
| organizer_id | BIGINT |
| location | VARCHAR |
| attendee_count | BIGINT |

### `pondville_residents` (100 rows)

| column | type |
| --- | --- |
| duck_id | BIGINT |
| name | VARCHAR |
| species | VARCHAR |
| home_pond | VARCHAR |
| age | BIGINT |
| occupation | VARCHAR |
| joined_date | DATE |

### `dbquacks_living_lake.pond_census`

A table in a MotherDuck share; its columns are not listed here.

### `dbquacks_marina_lake.ecosystem_readings`

A table in a MotherDuck share; its columns are not listed here.

## Data

The tables live in a DuckDB database: https://dbquacks.com/data/databases/motherduck_tutorial.duckdb

To query it outside the browser, attach it read-only:

```sql
ATTACH 'https://dbquacks.com/data/databases/motherduck_tutorial.duckdb' AS db (READ_ONLY);
USE db;
```

This level also queries MotherDuck shares, which need a MotherDuck token. On the page, the editor asks for one.

Play it in the browser: https://dbquacks.com/motherduck/17

## Reference

https://ducklake.select/docs/stable/duckdb/usage/time_travel