Advanced SQL: subqueries, WITH (CTEs) and window functions

The shop finally knows what it pays the wholesaler. Mama Ngo has two questions: "Was September better than August? And what do I really earn on, once the wholesaler is paid?" Neither fits in a simple query. The first compares a month with the previous one, the second combines tables grouped differently. It is SQL's new wall: the question that takes several steps. You learn to write it the way you think it, one named step after another, each checkable on its own. Along the way, three traps that raise no message. A division between integers that shows 0 % instead of 58 %. A NOT IN that returns zero rows because a single empty value lurks in the list. And a direct join between two tables that multiplies the amounts without warning. Each one is triggered on figures you know, so that you see they are wrong instead of believing them. You group by month and by day of the week, you calculate each product's margin with WITH, you classify with CASE, and you discover window functions: lag to compare each month with the previous one, rank to rank without grouping. The challenge: the most profitable product in each aisle. And a surprise in the figures: what fills the till is not what keeps the shop alive.