## Metadata * URL: [https://dev.mysql.com/doc/refman/5.7/en/semijoins.html](https://dev.mysql.com/doc/refman/5.7/en/semijoins.html) ## 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