merge maps take latest clickhouse

meegemap.sql

Handle the map as an array and then via ARRAY JOIN bring it into multiple rows, so you can use more complex SQL features. with argMax you could then get the latest value per key.
Then you need to regroup it again:

 SELECT source, integration_id, transaction_id, min(min_event_time), groupArray((field_key, field_value))::Map(String, String)
 FROM (
   SELECT source, integration_id, transaction_id, min(event_time) AS min_event_time, field.1 AS field_key, argMax(field.2, event_time) AS field_value
   FROM events
   ARRAY JOIN fields AS field
   GROUP BY source, integration_id, transaction_id, field.1
 )
 GROUP BY source, integration_id, transaction_id