CHAPTER 18: VISUAL LABS 18a AND 18b

These original exercises use the same synthetic Harbour dataset as the book.
The display/check runner opens companion/sql/harbour.db read-only.
If that file is missing, first follow Chapter17 and run sql/setup_database.py.

RUN
1. Open walkthrough.py in IDLE and choose Run > Run Module (F5).
2. Alternatively, open a terminal in this ch18 folder.
   Windows: py walkthrough.py
   macOS/Linux: python3 walkthrough.py
Use the Python command that already worked in your setup.
No network access or third-party package is required.

The .sql files hold inspectable questions. The supplied Python wrapper prints
results and checks the original expected values. Before editing, predict the
new result. After editing, compare it manually; an old assertion may now fail
because your intentional change needs a different expected answer. Do not
remove a failed check until you can explain the changed result.

EXPECTED RESULTS AND PRACTICE
18a returns campaign 2 only. 18b_grouped returns 1 and 2; removing parentheses in 18b_ungrouped adds campaign 4. Change/predict/check: lower the 18a minimum to 8000. Predict two rows before reading change_check.sql: campaigns 2 and 4. The inclusive boundary now admits Autumn webinar; the two non-email campaigns still fail.
