> First question
I didn't quite catch your meaning. There are some documents about using window in sql or table-api and it is worth to take a look.
> Second question
Group by window without other keys results a AllWindowedStream. Sql like bellow:
, TUMBLE_START(rowtime, INTERVAL '15' MINUTE)
, TUMBLE_END(rowtime, INTERVAL '15' MINUTE)
GROUP BY TUMBLE(rowtime, INTERVAL '15' MINUTE)
> Third question
You can print or sink data into external storages and compare the result.