Completion Counting only Once
You don't specify what "first" means (or provide any table structure or example data), but let's assume it's the earliest completion timestamp:

  SELECT entity, min(completion_ts)
    FROM your_table
GROUP BY entity


If you are just getting results for a single entity, you can simplify even more:

  SELECT min(completion_ts)
    FROM your_table
   WHERE entity = 'XXX'