To create a polygon around a pair of lat+lon coordinates I'd expect to be able to buffer the geogpoint:
e.g. ST_BUFFER(ST_GEOGPOINT(lat, lon), 1000)
This creates a circular polygon with a radius of 1000m.
Bigquery doesn't seem to have the buffer function, which seems like a really basic one - am I missing something? Thanks!
As for now, ST_BUFFER function is not supported in query engine yet. All available geography functions in Standard SQL you can see here. Additionally, you can think about using buffer function from the open source library Turf.JS to make buffers and use it in BigQuery:
jslibs.turf.ST_BUFFER(geometry_to_buffer GEOGRAPHY, radius NUMERIC, units STRING, steps NUMERIC)
Please follow carto tutorial to learn more about using this function.
ST_Buffer is now available (in preview) https://cloud.google.com/bigquery/docs/reference/standard-sql/geography_functions#st_buffer
There is also ST_BufferWithTolerance which allows one to specify approximation tolerance, rather than number of segments in polygon approximating the buffer.