How to compute a rolling period calculation in BigQuery?

In one of my previous posts, I've showcased using RANGE inside window function declarations in the context of computing a cumulative sum.

Today we're going to look at another example often found in the wild - computing a rolling period calculation. Let's look at an example.

We have customer order information and would like to compute a per customer rolling sum of the previous 60 days' worth of purchases.

How this would look in terms of SQL?

SUM(order_total) OVER (PARTITION BY customer_id ORDER BY UNIX_DATE(order_date) RANGE BETWEEN 59 PRECEDING AND CURRENT ROW) AS rolling_60_days_sum

Let's explain it:

We will start by taking a SUM of order_total with a window declaration

and partitioning by customer_id.

Next is ordering by our order_date, but since RANGE only accepts a single integer field, we'll need to transform it using UNIX_DATE. This will transform '2021-01-01' into 18628.

We can now use RANGE, setting the range between the 59 previous days and the current row. This way, if we have any gaps or duplicates in our order data (which is very likely), the calculation would still work, as opposed to the approach of using ROWS.

See below an illustration of how it all works.

```
SELECT
customer_id,
order_id,
order_total,
order_date,
SUM(order_total) OVER (PARTITION BY customer_id
ORDER BY UNIX_DATE(order_date)
RANGE BETWEEN 59 PRECEDING AND CURRENT ROW)
AS rolling_60_days_sum
FROM input_data
+-------------+----------+-------------+------------+---------------------+
| customer_id | order_id | order_total | order_date | rolling_60_days_sum |
+-------------+----------+-------------+------------+---------------------+
| Customer-1 | 10001 | 100 | 2021-01-01 | 100 |
| Customer-1 | 10003 | 75 | 2021-02-15 | 175 |
| Customer-1 | 10005 | 90 | 2021-03-12 | 165 |
| Customer-1 | 10001 | 100 | 2021-04-21 | 190 |
| Customer-1 | 10003 | 75 | 2021-05-12 | 175 |
| Customer-1 | 10005 | 90 | 2021-06-23 | 165 |
| Customer-2 | 10002 | 80 | 2021-01-01 | 80 |
| Customer-2 | 10004 | 120 | 2021-02-04 | 200 |
| Customer-2 | 10006 | 50 | 2021-03-05 | 170 |
| Customer-2 | 10002 | 80 | 2021-04-11 | 130 |
| Customer-2 | 10004 | 120 | 2021-05-12 | 200 |
| Customer-2 | 10006 | 50 | 2021-06-30 | 170 |
+-------------+----------+-------------+------------+---------------------+
```

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