I know the course is due a refresh, but the question:-
Consider an Aggregation Pipeline using the new $merge stage that outputs to the employee_data collection.
If we are not expecting to find any matching documents in the employee_data collection, which of the following stages should we use?
Maybe a clearer second line of the question would be.
If a match is found, then the operation should fail. Which of the following stages best matches that requirement?
In the question, the action to be taken in case of a match is not stipulated, though it is hinted at, so I assumed you would want it to fail.
Again, lets make it less about a persons knowledge of English
PS - Nice to learn about merge though.