The SQL Standard defines several ways to perform a JOIN, the most common of which is the INNER JOIN. Imagine for a moment that you’re linking students and the classes for which they registered. You might have some students who have been accepted to attend the school but have not yet registered for any classes, and you might also have some classes that are on the schedule but do not yet have any students registered.

An INNER JOIN between the Students table and the Classes table returns rows in the Students table linked with the related rows in the Classes table (via the Student_Schedules table)—but it returns neither students who have not yet registered for any classes nor any classes for which no student is registered. An INNER ...

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