*Running sums are useful in situations where you want to get a total, but you also want to see how much each entry of a list contributes to the total. So each entry is accompanied by a partial sum, up to that point of the list. While getting a total in SQL is trivial, getting a running sum requires the use of window functions.*

*Problem: Show the sum of 2020 income, including a month by month running sum.*

```
SELECT
month,
SUM (income) OVER (ORDER BY month)
AS running_sum
FROM sales
WHERE year = 2020;
```

We’re summing the income column (`SUM(income)`

) ordered by month (`OVER (ORDER BY month)`

) and we restrict this to the year 2020 (`WHERE year = 2020`

).

We could invert the order of the running sum as well, with `ORDER BY month DESC`

.

This is a trivial running sum case. There are more complex cases, but we’re leaving them to future articles.

## Reference

Other articles: