text: -- e.g1
  DELETE
  FROM
    Products
  WHERE
    ProductID NOT IN
    (
      SELECT
        ProductID
      FROM
        Sales
    )
  ;
  
  -- practice1
  DELETE
  FROM
    Salary
  WHERE
    EmployeeID  NOT IN
    (
    SELECT
      EmployeeID
    FROM
      Sales
    )
  ;
  
  -- practice2
  DELETE
  FROM
  Products
  WHERE ProductID NOT IN
  (
    SELECT
      ProductID
    FROM
      Sales
    GROUP BY
      ProductID
    HAVING
      SUM(Quantity) >= 20
  )
  ;
  
  -- practice3
  DELETE
  FROM
    Customers
  WHERE
    CustomerID NOT IN
    (
      SELECT
        CustomerID
      FROM
        Sales
    )
  ;
  
  -- practice4
  DELETE
  FROM
  Employees
  WHERE
  EmployeeID NOT IN
  (
    SELECT
    EmployeeID
    FROM
    Sales
    GROUP BY
    EmployeeID
    HAVING
      COUNT(*) > 5
  );
  
  -- practice5
  DELETE
  FROM
  Sales
  WHERE
  EmployeeID IN
  (
    SELECT
    EmployeeID
    FROM
    BelongTo
    WHERE
    EndDate IS NULL
    AND
    DepartmentID = 3
  )
  ;
  