Here is an example DELETE statement that will remove all employees in the Sales department who have a salary less than the average salary of all employees in the department:

```
DELETE FROM employee
WHERE DepartmentID = 'Sales' AND Salary < (
    SELECT AVG(salary) FROM employee WHERE DepartmentID = 'Sales'
)
``` 

Make sure to replace 'employee' with the actual name of your table, and 'DepartmentID' and 'Salary' with the actual names of the columns in your table.
Data una tabella degli stipendi dei dipendenti con le colonne EmployeeID, Salary e DepartmentID, scrivere un'istruzione DELETE per rimuovere tutti i dipendenti del reparto Vendite che hanno uno stipendio inferiore allo stipendio medio di tutti i dipendenti del dipartimento.