explain upsert

content/post/flink-sql-wrangling.adoc

@@ -776,7 +776,7 @@ SHOW CREATE TABLE `measures_with_pk`;
 The PK is defined, yes—but I think there are two problems here:
 
 1. `'kafka.retention.time' = '7 d'`: If there's no new data pulled into the source topic (`flood-monitoring-measures`) for a week then the data will age out of this table, and we don't want that (https://docs.confluent.io/cloud/current/flink/reference/sql-examples.html#table-with-infinite-retention-time[ref]).
-2. `'changelog.mode' = 'append',` (https://docs.confluent.io/cloud/current/flink/reference/sql-examples.html#create-with-different-changelog-modes[ref])
+2. `'changelog.mode' = 'append',` (https://docs.confluent.io/cloud/current/flink/reference/sql-examples.html#create-with-different-changelog-modes[ref]): as this is a dimension table, we don't want to _add_ (append) data to it, but update existing values for a key or insert them if they don't exist—which is what `upsert` does.
 
 Let's try changing these.