A join is how you ask one question across two tables. The database keeps patients in one table and appointments in another, because storing the patient's name on every appointment row would mean fixing it in ten places when it's misspelled. A join puts them back together for the length of one query. That's the whole idea. The part that takes practice is that there are several kinds of joins, and they disagree about what to do with rows that have no partner on the other side. Pick the wrong on...