Correlated Subqueries II

Hey guys, I’m currently working on this topic. I found one thing that disturbs me a lot.

From the example, the line goes:
AND f.carrier=flights.carrier) + 1

Since the query is for viewing flights by carrier, flight id, and sequence number, I’m not quite understand that why there is a condition when the id is smaller than others? And what is the ‘+1’ means?

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