The baseline paid total is 300: North 220, South 80 and West 0. A correct customer LEFT JOIN retains C4 with count 0 and value 0. Adding paid order O7 for 20 changes only West and the grand total, now 320.

Critical check: The paid total reconciles and the zero-order customer is retained without counting a phantom order.

Final project: Submit schema, seed data, detail and summary queries, expected results and a README. Demonstrate duplicate-ID rejection and the O7 refresh scenario. A reviewer must be able to recreate the database from scratch.