Explain a SQL Function

Explain a SQL code snippet.

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Prompt Freeform

Explain the function of CROSS APPLY in the code below:

SELECT customer.CustomerId, customer.CustomerName, ord.*
FROM customers customer
CROSS APPLY
(
    SELECT SUM(order.quantity) as TotalQuantity
    FROM orders order
    WHERE order.CustomerId = customer.CustomerId
) ord;

Response

The CROSS APPLY function in this SQL code acts as a correlated subquery that applies the results of the subquery to each row in the customers table. Here's a breakdown:

Outer Query: The main query selects CustomerId and CustomerName from the customers table, aliased as customer.
CROSS APPLY: This keyword introduces the correlated subquery. It instructs the database to execute the subquery for each row in the customers table.
Subquery: The subquery calculates the TotalQuantity for each customer by:
Selecting the sum of quantity from the orders table, aliased as order.
Filtering the orders table based on the CustomerId from the current row in the customers table.
Result: The CROSS APPLY returns a single row for each customer, containing the CustomerId, CustomerName, and the calculated TotalQuantity.

In essence, CROSS APPLY allows you to dynamically generate a new column (TotalQuantity) for each customer based on the results of the subquery, effectively joining the customers table with the aggregated order data.

Example:

Let's say you have the following data:

Customers:

CustomerId	CustomerName
1	John Doe
2	Jane Smith

Orders:

OrderId	CustomerId	Quantity
1	1	5
2	1	3
3	2	2

The query would return the following result:

CustomerId	CustomerName	TotalQuantity
1	John Doe	8
2	Jane Smith	2

Note:

CROSS APPLY is similar to OUTER APPLY, but OUTER APPLY returns a row even if the subquery doesn't return any results.
In this specific example, you could achieve the same result using a JOIN with a GROUP BY clause. However, CROSS APPLY can be more flexible and efficient in certain scenarios, especially when dealing with complex subqueries.

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