Each task here is a small job you might get at work: a new table for a feature, rows to add, a value to fix, rows to clean out. The check runs your script on the exercise's starting tables and compares the table afterwards with the expected one, so you can run as often as you like.
Before an UPDATE or DELETE, try the WHERE in a SELECT to see which rows it picks. Remember that text matches exactly, NULL needs IS NULL, and an apostrophe in text is written twice.