The questionable claim
An order worth EUR100 has two item rows. After joining order totals to items, SUM(order_total) shows EUR200.
Investigate
Identify the unit of analysis, denominator, population or information boundary. What extra evidence would be needed? Write a corrected statement first.
Diagnosis and repair
Order totals are repeated at item grain. Aggregate line amounts or aggregate each side to the required order grain before joining. Reconcile unique order count and pre/post amounts. DISTINCT on a monetary amount can wrongly merge different orders with equal values.
Prevention
Add a test or review question that catches the error before publication. Save the original and corrected explanation so your reasoning is auditable.
Try the activity before reading its reference. Keep your sources, commands and assumptions with your work. Companion activities are unsaved self-practice and do not award a certificate or change lesson progress.