Problem

Given a table of transactions, write a SQL query to find the total value of all transactions for each day.
Example input:
| id | date       | value |
|----|------------|-------|
| 1  | 2019-01-01 | 100   |
| 2  | 2019-01-02 | 200   |
| 3  | 2019-01-03 | 300   |
Example output:
| date       | total_value |
|------------|-------------|
| 2019-01-01 | 100         |
| 2019-01-02 | 200         |
| 2019-01-03 | 300         |

Solution

SELECT date, SUM(value) AS total_value
FROM transactions
GROUP BY date;

/*

The solution above is optimal because it uses the GROUP BY clause to group the transactions by date.
The SUM function is used to sum the values of each group.

*/