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Query Activity appends Data Extension instead of overwriting information

Knowledge Article Number 000220946
A Query Activity that's set to overwrite the Data Extension doesn't overwrite as expected. Instead, information is appended in the Data Extension. This is because the external key for the Data Extension has been modified since the creation of the Query Activity. If the name of the Data Extension doesn't change, but the customer key of the Data Extension does, then the erasure action of the Query Activity will silently fail. This will cause the previous data to be retained and the new data to append to the existing data. 

Update Customer Key 

The solution is to repoint and re-save the Query Activity to update the customer key for the Data Extension. Here's how: 

1. Log in to your MC Account.
2. Navigate within the Email App to the Interactions Tab | Activities | Query.
3. Click the name of the Query Definition experiencing the issue.
4. Under the Target section, re-select the Target Data Extension by name.
5. Click Save.
6. Manually re-initiate the Query Activity, or let it run as scheduled if within an Automation.

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