A Common Table Expression (CTE) is a temporary, named result set that you can reference within a SQL query. It is defined using the WITH keyword and exists only for the duration of the query it belongs to. CTEs act like a short-lived view or a derived table, allowing you to break down complex queries into smaller, reusable parts. For example, you might create a CTE to calculate aggregated data before joining it with other tables in the main query. CTEs are particularly useful for improving readability and simplifying nested subqueries.
CTEs offer two main benefits: modularity and recursion. By isolating parts of a query into CTEs, you can structure code more logically, making it easier to debug or modify. For instance, if you need to filter a dataset before applying multiple transformations, a CTE can hold the filtered data, and subsequent steps can reference it cleanly. Recursive CTEs, a specialized form, enable querying hierarchical data (like organizational charts or category trees) by repeatedly executing a query until a condition is met. This avoids the need for procedural loops in SQL, though recursion is supported only in databases like PostgreSQL, SQL Server, or Oracle.
To illustrate, consider a scenario where you need to calculate total sales per region and then compare each region’s sales to the average. Without a CTE, you might nest aggregation functions or repeat calculations. With a CTE, you can first compute the totals in a named block (WITH region_sales AS (...)), then reference it in the main query to perform comparisons. This approach reduces redundancy and keeps the logic centralized. While CTEs don’t always improve performance, their clarity and maintainability make them a practical tool for organizing complex operations, especially when working with multi-step data transformations or recursive relationships.