sql_query.txt

# In this schema, each invoice is guaranteed to have a CustomerId, so an inner join allows us to retrieve the customer’s name associated with each invoice.
#The ReferredBy column in the Customers table refers to another customer within the same table.
#To ensure we list all invoices, even those without a referrer, we use a LEFT JOIN to check for a referring customer and retrieve their name if available.
#This way, invoices without a referrer will still appear in the results, with the ReferrerName shown as NULL.

SELECT
    i.Id AS InvoiceId,
    i.BillingDate,
    c1.Name AS CustomerName,
    c2.Name AS ReferrerName
FROM
    Invoices i
INNER JOIN
    Customers c1 ON i.CustomerId = c1.Id
LEFT JOIN
    Customers c2 on c1.ReferredBy = c2.Id
ORDER BY i.BillingDate ASC