As queries grow, deeply nested subqueries become hard to read and harder to debug. A Common Table Expression, or CTE, solves this by letting you define a named, temporary result set at the top of a query using the WITH keyword, then reference it by name below. It turns an unreadable pyramid of nesting into a clean, top-to-bottom sequence of named steps.
A CTE is essentially a derived table with a name and better ergonomics. Instead of embedding a subquery inside FROM and aliasing it inline, you declare it once with WITH and use its name like a real table. This separation of definition from use makes complex logic readable, because each stage is named, defined in isolation, and composed in the order a human actually thinks about the problem.
CTEs are among the most loved features in modern SQL because they make sophisticated queries maintainable. You can chain several CTEs so each builds on the last, reuse a CTE's result multiple times in the main query, and read the whole thing as a logical pipeline. Mastering CTEs is the difference between writing clever queries and writing queries other people, including future you, can understand.