Common Table Expressions (CTEs) are a feature in SQL that allow you to define a temporary named result set that can be used within a SELECT, INSERT, UPDATE, or DELETE statement. They provide a way to simplify complex queries and improve readability by breaking down a query into smaller, reusable parts.

CTEs can be thought of as named subqueries, as they can be used to encapsulate a query and refer to it later in the same statement. The key difference between subqueries and CTEs is that subqueries are unnamed and can only be used within the same query, while CTEs can be named and can be referred to multiple times within a query.

You should use CTEs when you have a complex query that needs to be broken down into smaller, more manageable parts. They can also be used to improve the readability and maintainability of a query by giving meaningful names to intermediate results. Additionally, CTEs can provide performance benefits, as the query optimizer may be able to use the results of a CTE multiple times without having to recompute them each time.

Here's an example of a query that uses a CTE:

vbnet

WITH TotalSales AS (
   SELECT CustomerID, SUM(Amount) AS TotalAmount
   FROM Orders
   GROUP BY CustomerID
)
SELECT Customers.Name, TotalAmount
FROM Customers
JOIN TotalSales ON Customers.CustomerID = TotalSales.CustomerID

In this example, the CTE named "TotalSales" is used to compute the total sales for each customer. The results of the CTE are then used in the main query to join with the customers table and retrieve the customer name and total sales.
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