For example, how can we use a CTE to simplify joins between tables?

A Common Table Expression (CTE) is a temporary result set defined with the WITH clause. It can be referenced multiple times in the main query, making complex join logic easier to read and maintain. For instance, you can first compute a filtered or aggregated view of a table in a CTE, then join that CTE with other tables. This avoids repeating sub‑queries, reduces code duplication, and can improve query planner optimization. Example: WITH sales AS (SELECT * FROM orders WHERE order_date > '2023-01-01') SELECT s.id, c.name FROM sales s JOIN customers c ON s.customer_id = c.id; The CTE ‘sales’ is reused in the join, simplifying the overall query.