COUNT in SQL Server: rows, values, and distinct people

Three different questions. See also <a href="/guides/count-vs-count-distinct">COUNT vs COUNT DISTINCT</a>.

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SELECT COUNT(*) AS orders,
       COUNT(customer_id) AS orders_with_customer,
       COUNT(DISTINCT customer_id) AS customers
FROM Orders
WHERE order_date >= DATEADD(day, -7, CAST(GETDATE() AS date))
  AND order_date <  CAST(GETDATE() AS date);