INSERT adds rows, UPDATE changes matching rows and DELETE removes matching rows. A WHERE clause is a write boundary. If you omit it, an UPDATE or DELETE can affect the entire table. Read the predicate against the source before you run a change, then read back the table afterward.
Aster Supply keeps a small working table for these drills. Its bronze data stays intact. You will correct one order, append a new record and remove canceled orders. NULL amounts need an explicit IS NULL predicate just as they do in SELECT queries. Delta commits each successful write as a new version. That history helps recovery, but it doesn't make careless predicates harmless: downstream readers can consume a bad version before anyone restores it. An append should preserve existing rows instead of rebuilding a table from only the new batch.