Use TREAT to Access Attributes of Object Subtypes

The TREAT function comes in very handy when working with an object type hierarchy, and you need to access attributes or methods of a subtype of a row or column's declared type. This topic was covered in a PL/SQL Challenge quiz offered in March 2016. 

Suppose I have the following type hierarchy and  I use them as column types in my meals table:

CREATE TYPE food_t AS OBJECT (
   name VARCHAR2 (100),
   food_group VARCHAR2 (100),
   grown_in VARCHAR2 (100)
)
   NOT FINAL;
/

CREATE TYPE dessert_t UNDER food_t (
      contains_chocolate CHAR (1),
      year_created NUMBER (4)
   )
   NOT FINAL;
/

CREATE TYPE cake_t UNDER dessert_t (
      diameter NUMBER,
      inscription VARCHAR2 (200)
   );
/

CREATE TABLE meals
(
   served_on     DATE,
   appetizer     food_t,
   main_course   food_t,
   dessert       dessert_t
);

I then insert some rows into the table:

BEGIN
   INSERT INTO meals
        VALUES (SYSDATE + 1,
           food_t ('Shrimp cocktail', 'PROTEIN', 'Ocean'),
           food_t ('Stir fry tofu', 'PROTEIN', 'Vat'),
           cake_t ('Apple Pie',
'FRUIT', 'Baker''s Square',
'N', 2001, 8, NULL));

   INSERT INTO meals
        VALUES (SYSDATE + 1,
                food_t ('Fried Calamari', 'PROTEIN', 'Ocean'),
                dessert_t ('Butter cookie',
'CARBOHYDRATE', 'Oven', 'N', 2001),
                cake_t ('French Silk Pie',
'CARBOHYDRATE', 'Baker''s Square',
                   Y', 2001, 6, 'To My Favorite Frenchman'));

   INSERT INTO meals
        VALUES (SYSDATE + 1,
                food_t ('Fried Calamari', 'PROTEIN', 'Ocean'),
                cake_t ('French Silk Pie',
'CARBOHYDRATE', 'Baker''s Square',
'Y', 2001, 6, 'To My Favorite Frenchman'),
                dessert_t ('Butter cookie',
'CARBOHYDRATE', 'Oven', 'N', 2001));

   COMMIT;
END;
/

Notice that even though appetizer and main_course are defined as food_t, I can assign dessert_t and cake_t instances to those columns, because object types support substitutability (the best way to understand that is: every dessert is a food, but not every food is a dessert).

Let's take a look at some of the ways I can use TREAT.

1. I want to find all the meals in which the main course is actually a dessert.

SELECT *
  FROM meals
 WHERE TREAT (main_course AS dessert_t) IS NOT NULL

2. Show whether or not those dessert-centric meals contain chocolate. First with PL/SQL:

DECLARE
   l_dessert   dessert_t;
BEGIN
   FOR rec IN (

      SELECT * FROM meals
       WHERE TREAT (main_course AS dessert_t) IS NOT NULL)
   LOOP
      l_dessert := TREAT (rec.main_course AS dessert_t);

      DBMS_OUTPUT.put_line (
         rec.main_course.name || '-' || 
         l_dessert.contains_chocolate);
   END LOOP;
END;
/

And now with "pure" SQL:

SELECT TREAT (m.main_course AS dessert_t).contains_chocolate
  FROM meals m
 WHERE TREAT (main_course AS dessert_t) IS NOT NULL

The thing to realize in both these cases is that even though I have identified only those meals for which the main course is a dessert, I still must explicitly TREAT or narrow the main_course column to dessert_t, before I will be able to reference the contains_chocolate attribute.

If I forget the TREAT in the SELECT list, such as:

SELECT m.main_course.contains_chocolate
  FROM meals m
 WHERE TREAT (main_course AS dessert_t) IS NOT NULL

I will see this error:

ORA-00904: "M"."MAIN_COURSE"."CONTAINS_CHOCOLATE": invalid identifier

3. Set to NULL any desserts that are not cakes.

UPDATE meal
   SET dessert = TREAT (dessert AS cake_t);