Very good!

Now, it might be tempting to use window functions inside the WHERE clause, as shown below:

SELECT
  FirstName,
  LastName,
  Salary,
  AVG(Salary) OVER() AS AvgSalary
FROM Employee
WHERE Salary > AVG(Salary) OVER();

However, when you run this query, you'll get an error message. You cannot put window functions in WHERE. Why? The window function is applied after the rows are selected. If the window functions are in a WHERE clause, you get a circular dependency: in order to compute the window function, you have to filter the rows with WHERE, which requires the computation of the window function.