-- SQL Server query to retrieve staff information and their total quantity of items sold.

-- Selects the staff's first name, last name, email, and calculates the sum of quantities sold for each staff member.
SELECT 
    staffs.first_name AS 'First Name',  -- Staff's first name.
    staffs.last_name AS 'Surname',      -- Staff's last name.
    staffs.email AS 'Email',            -- Staff's email address.
    SUM(order_items.quantity) AS [Total quantity sold] -- Total quantity of items sold by the staff member.
FROM 
    BikeStores..orders -- Joins the 'orders' table...
JOIN
    BikeStores..order_items ON orders.order_id = order_items.order_id -- ...to the 'order_items' table using the order ID.
JOIN 
    BikeStores..staffs ON orders.staff_id = staffs.staff_id -- ...and to the 'staffs' table using the staff ID.
GROUP BY 
    staffs.first_name,  -- Groups the results by staff's first name...
    staffs.last_name,  -- ...last name...
    staffs.email       -- ...and email address to aggregate sales per staff member.
ORDER BY 
    [Total quantity sold] -- Orders the results in ascending order based on the total quantity sold.