BigQuery SQL for 28-day sliding window aggregate (without writing 28 lines of SQL)

Question

I'm trying to compute a 28 day moving sum in BigQuery using the LAG function.

The top answer to this question

Bigquery SQL for sliding window aggregate

from Felipe Hoffa indicates that that you can use the LAG function. An example of this would be:

SELECT
    spend + spend_lagged_1day + spend_lagged_2day + spend_lagged_3day + ... +  spend_lagged_27day as spend_28_day_sum,
    user,
    date
FROM (
  SELECT spend,
         LAG(spend, 1) OVER (PARTITION BY user ORDER BY date) spend_lagged_1day,
         LAG(spend, 2) OVER (PARTITION BY user ORDER BY date) spend_lagged_2day,
         LAG(spend, 3) OVER (PARTITION BY user ORDER BY date) spend_lagged_3day,
         ...
         LAG(spend, 28) OVER (PARTITION BY user ORDER BY date) spend_lagged_day,
         user,
         date
  FROM user_spend
)

Is there a way to do this without having to write out 28 lines of SQL!

Answer 1

The BigQuery documentation doesn't do a good job of explaining the complexity of window functions that the tool supports because it doesn't specify what expressions can appear after ROWS or RANGE. It actually supports the SQL 2003 standard for window functions, which you can find documented other places on the web, such as here.

That means you can get the effect you want with a single window function. The range is 27 because it's how many rows before the current one to include in the sum.

SELECT spend,
       SUM(spend) OVER (PARTITION BY user ORDER BY date ROWS BETWEEN 27 PRECEDING AND CURRENT ROW),
       user,
       date
FROM user_spend;

A RANGE bound can also be extremely useful. If your table was missing dates for some user, then 27 PRECEDING rows would go back more than 27 days, but RANGE will produce a window based on the date values themselves. In the following query, the date field is a BigQuery TIMESTAMP and the range is specified in microseconds. I'd advise that whenever you do date math like this in BigQuery, you test it thoroughly to make sure it's giving you the expected answer.

SELECT spend,
       SUM(spend) OVER (PARTITION BY user ORDER BY date RANGE BETWEEN 27 * 24 * 60 * 60 * 1000000 PRECEDING AND CURRENT ROW),
       user,
       date
FROM user_spend;