When Should You Use a Common Table Expression (CTE) Instead of a Derived Table?

When is a Common Table Expression (CTE) Necessary?

While derived tables may appear to suffice in all scenarios, Common Table Expressions (CTEs) offer distinct advantages under certain circumstances.

Limitations of Derived and Temp Tables

Consider a scenario where data needs to be joined multiple times. Using regular SELECT statements or derived tables would result in replicating the join syntax for each instance, increasing code verbosity. However, CTEs allow you to define a reusable table that can be referenced multiple times in the query.

Example

Suppose you have a "Customers" table and want to find customers who have placed multiple orders.

Using CTE:

WITH CustomerOrders AS (
    SELECT CustomerID, COUNT(*) AS OrderCount
    FROM Orders
    GROUP BY CustomerID
)

SELECT *
FROM Customers
JOIN CustomerOrders ON Customers.CustomerID = CustomerOrders.CustomerID
WHERE OrderCount > 1;

Using Derived Table:

(SELECT CustomerID, COUNT(*) AS OrderCount
FROM Orders
GROUP BY CustomerID) AS CustomerOrders

SELECT *
FROM Customers
JOIN CustomerOrders ON Customers.CustomerID = CustomerOrders.CustomerID
WHERE OrderCount > 1;

As you can see, the CTE approach eliminates the need to define the derived table alias ("CustomerOrders") twice.

Other CTE Benefits

Apart from reusability, CTEs offer additional benefits:

• Recursion: Recursion can be implemented using CTEs, allowing for complex queries that traverse hierarchical data structures.
• Temporary storage: CTEs provide temporary storage for intermediate query results, without the need to create explicit temp tables.
• Grouping by derived columns: CTEs enable grouping by columns derived from scalar subqueries or non-deterministic functions, which is not possible with derived tables.

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