Join impressions and sessions to find average revenue

Question

Write a query joining impressions and sessions to produce the combined average revenue.

Codemia · Accepted Answer

In this problem, we have two datasets: `impressions` and `sessions`. The `impressions` table contains records of ad impressions along with their associated revenue, while the `sessions` table tracks user sessions on the website. To calculate the combined average revenue, we need to join these two datasets based on a common key, likely a session ID or timestamp, then compute the average revenue from the impressions for those sessions.

1. **Identify Common Keys**: Determine the key to join `impressions` and `sessions`. This is usually `session_id`.  
2. **Join Tables**: Use an INNER JOIN to connect both tables, ensuring that we only consider sessions that have impressions.  
3. **Compute Average Revenue**: Utilize the AVG() function to compute the average revenue from the joined dataset.  
4. **Group By**: If necessary, group by session attributes to ensure we get the average revenue per session or overall average.  
5. **Handle NULLs**: Ensure that any NULL revenues are handled appropriately, possibly using COALESCE to substitute them with 0 or filtering them out.

```sql
SELECT AVG(i.revenue) AS average_revenue
FROM impressions i
JOIN sessions s ON i.session_id = s.session_id;
```

To optimize this query for large datasets, consider the following:  
1. **Indexes**: Ensure that there are indexes on `session_id` in both the `impressions` and `sessions` tables to speed up the join operation.  
2. **Selective Filtering**: If there are additional filters (e.g., date ranges for sessions), apply them early in the query to reduce the dataset size before performing the join.  
3. **Materialized Views**: If this query is run frequently, consider creating a materialized view that pre-aggregates the revenue data.

1. **NULL Values**: If any records in the `impressions` table have NULL revenue, they could skew the average. Use `COALESCE(i.revenue, 0)` to handle these.  
2. **Duplicates**: If there are duplicate session IDs in either table, it could result in inflated revenue averages. Consider using DISTINCT in the select statement or handle duplicates in the data cleaning phase.  
3. **No Matches**: If there are sessions with no impressions, they won't be included in the average. Ensure that this aligns with the business logic required for your analysis.

Join impressions and sessions to find average revenue

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