Chapter 8: Subqueries
Scalar, correlated, table subqueries, and when nesting improves clarity.
Chapter 9: CTEs
Readable multi-step SQL using common table expressions and recursive patterns.
Chapter 7: Aggregation
Every query so far has answered questions about individual rows: which products, which customers, which orders. Most business questions don't work that way. "How many products do we carry?" "What's our average order value?" "Which category sells the most?" None of these point at a single row; they summarize across many rows at once. That summarizing is aggregation, and it's the most direct path from raw data to something a manager can act on.