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

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

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