I'm trying to understand why Marketing Cloud requires a
LEFT JOIN where one key is NULL to add users to an existing Data Extension when we choose
UPDATE as the processing...?
For clarity, the error received is:
Query failed during execution. Error: Violation of PRIMARY KEY constraint.
I know that the query fails because the key already exists, and the
LEFT JOIN eliminates a duplicate but why is this necessary when
UPDATE is chosen since that is supposed to update records if they already exist, and insert when the record is new.
Can anyone shed light on why?