Chapter 7
A Simple Case Study
Pages 166-67

CREATE TABLE prices
  ( product_code  VARCHAR2(10) NOT NULL
  , date_from     DATE         NOT NULL
  , date_to       DATE
  , price         NUMBER       NOT NULL
  , CONSTRAINT prices_date_range CHECK (date_from < date_to)
  );

CREATE OR REPLACE FUNCTION price_at_date
    ( p_code IN price.product_code%TYPE
    , p_when IN DATE
    ) RETURNS NUMBER IS
  p_price NUMBER;
BEGIN

  SELECT price 
    INTO p_price
    FROM prices
   WHERE product_code = p_code 
    AND  date_from   >= p_when
    AND  date_to     <= NVL(p_when, 
                         TO_DATE('.1-DEC-4712'. 'DD-MON-YYYY'.

  RETURN p_price;

EXCEPTION
  WHEN no_data_found THEN
  RETURN 0;

END price_at_date;