Let's first define what we mean by a "churned customer." In our business, we'll define a churned customer as a customer that hasn't placed an order in more than 30 days. This definition will be a starting point – we'll use other criteria in some of the other examples too. In your own business, you can use criteria that fit your business better.

The simplest question related to customer churn is:
As of today, how many churned customers are there in total?
Here's a query that will provide the answer:

SELECT COUNT(*) AS ChurnedCustomers
FROM Customers
WHERE DATEDIFF(Day, LastOrderDate, GETDATE()) > 30;

In the WHERE clause, we used the DATEDIFF() function to find the number of days between LastOrderDate and today. To get the current date, we used the GETDATE() function.