title: Google BQ - how to upsert existing data in tables?
body: <p>I'm using Python client library for loading data in BigQuery tables. I need to update some changed rows in those tables. But I couldn't figure out how to correctly update them? I want some similar <code>UPSERT</code> function - insert row only if its not exists, otherwise - update existing row.</p>
  
  <p>Is it the right way to use a special field with checksum in tables (and compare sum in loading process)? If there is a good idea, how to solve this with Python client? (As I know, it can't update existing data)</p>
  
  <p>Please explain me, what's the best practice?</p>