Ein SQL-Code-Snippet erklären.
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Freiform-Prompt
Explain the function of `CROSS APPLY` in the code below:
```sql
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;
```
Antwort
The `CROSS APPLY` operator in the provided SQL code performs a lateral join,
which allows you to access data from a related table for each row in the outer
table. In this case, the outer table is the `customers` table, and the related
table is the `orders` table.
The `CROSS APPLY` operator is used to calculate the total quantity of orders for
each customer. The subquery within the `CROSS APPLY` operator calculates the sum
of the `quantity` column from the `orders` table for each customer. The result
of the subquery is then assigned to the alias `ord`, which is used to access the
calculated total quantity in the outer query.
The output of the query will be a table that contains the following columns:
* `CustomerId`
* `CustomerName`
* `TotalQuantity`
For each row in the `customers` table, the query will calculate the total
quantity of orders placed by that customer.
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