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Introduction
Providing detailed information and counting objects
Calculating metrics for multiple business objects
5. Computations for multiple objects
Understanding the difference between various count metrics
Summary

Instruction

Well done! In business reports, we often want to calculate certain metrics for multiple business objects at the same time. For instance:

SELECT
  o.order_id,
  COUNT(*) AS order_items_count
FROM orders o
JOIN order_items oi
  ON o.order_id = oi.order_id
WHERE o.order_id BETWEEN 10200 AND 10300
GROUP BY o.order_id;

In the query above, we count the number of items in each order with an order_id in the specified range. By joining the orders and order_items tables, we can display information about a single order item and its parent order on the same row. Then, we group all rows by their parent order and use COUNT(*) to find the number of items in each order.

Exercise

Show each supplier_id alongside the company_name and the number of products they supply (as the products_count column). Use the products and suppliers tables.