Explain calling a PL/SQL function defined in the WITH clause of a query

Since 12.1 a SELECT can define a function in its WITH clause: WITH FUNCTION f(...) RETURN ... IS BEGIN ... END; SELECT f(col) FROM ... . The function is visible only to that statement and runs with less context-switch overhead than a stored function — useful for a one-off transformation in a report. The statement must end with the slash-terminated style in tools that otherwise treat ';' as the terminator. A stored function can get similar speed with the UDF pragma.

WITH
  FUNCTION annual_cost(p_monthly IN NUMBER, p_load_pct IN NUMBER) RETURN NUMBER IS
  BEGIN
    RETURN ROUND(p_monthly * 12 * (1 + p_load_pct / 100), 2);
  END;
SELECT employee_id,
       annual_cost(salary, 30) AS fully_loaded_annual
FROM   employees
WHERE  department_id = 30;
/