Problem

Write a SQL query that returns the average transaction value for each day.
Example input:
| day | transaction_value |
| --- | ----------------- |
| 1   | 10                |
| 2   | 20                |
| 3   | 30                |
Example output:
| day | average_value |
| --- | ------------- |
| 1   | 10            |
| 2   | 15            |
| 3   | 20            |

Solution

SELECT day, AVG(transaction_value) AS average_value
FROM table
GROUP BY day;

/*

The solution above is optimal because it uses the AVG function to calculate the average value for each day.

*/