Design a Data Analysis Agent

An analyst asks an agent, "What was our revenue last month?" The agent writes a query, the database runs it without an error, and the agent replies with 1,240,000 dollars. The finance report for the same month says 1,010,000 dollars.

Every part of this run worked. The query was valid, the tables were real, and the arithmetic inside the database was correct. Where did the extra 230,000 dollars come from, and how could a design notice it before the analyst does?

The answer appears below, and it explains the main difficulty of this product