Question

I have created the following in Oracle PSL/SQL:

I have created a type Animal.
This has the attributes: Name, Age

I have created the type Dog. This inherits from type Animal
The only extra field in Dogis a nested table of references of place lived. I want to store all instances of Dogin the Animal table.

This is the bit I am confused about: When creating the Animal table of type Animal, how do I create the nested table for places lived? There is no field in Animal for this, only in Dog.

Answer 1

“When creating the Animal table of type Animal, how do I create the
nested table for “places lived”? when there is no field in Animal for
this, only in Dog.”

This is the mystery of inheritance. A table built from the type ANIMAL actually has columns to support the attributes of its sub-types. However, they are only accessible when we explicitly use the DOG sub-type.

Here is your data structure.

create or replace type animal_t as object 
  ( name varchar2(10)
    , age number (3,0))
not final;
/

create or replace type places_nt as table of varchar2(20)
/

create or replace type dog_t under animal_t
 ( residence_history places_nt)
/

create table animals of animal_t;

To create a record for a goldfish we do this:

insert into animals
  values (animal_t('BOB', 7))
/

To create a record for a dog we need to do this:

insert into animals
  values (dog_t('FIDO', 12, places_nt('Balham', 'Tooting')))
/

This query will just select the generic columns:

SQL> select * from animals
  2  /

NAME              AGE
---------- ----------
BOB                 7
FIDO               12

SQL>

To gain sight of the details pertaining to dogs we need to cast the record to the relevant sub-type:

SQL> select treat(value(a) as dog_t)
  2  from animals a
  3  where value(a) is of (dog_t)
  4  /

TREAT(VALUE(A)ASDOG_T)(NAME, AGE, RESIDENCE_HISTORY)
--------------------------------------------------------------------------------
DOG_T('FIDO', 12, PLACES_NT('Balham', 'Tooting'))

SQL>

There is an entire book in the Oracle documentation devoted to its Object-Relational features. Find out more.

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Note: I have used an object table here simply because it is easy to illustrate how nested tables work. I do not recommend using types for data storage: OO is a programming paradigm and should only be used for writing programs. Data should always be persisted in relational structures.