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Highlights
- the subquery to return only one instance of each class number from the roster table. In this case, the query can use a semijoin; that is, an operation that returns only one instance of each row in class that is matched by rows in roster.
- MySQL converts it to a semijoin and makes a cost-based choice from these strategies:
- Convert the subquery to a join,
- and run the query as an inner join between subquery tables