Oracle 18c

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CREATE TYPE client IS OBJECT(
  id         INT,
  first_name VARCHAR2(200),
  last_name  VARCHAR2(200)
) NOT INSTANTIABLE NOT FINAL;
CREATE TYPE single UNDER client (
  date_of_birth DATE
) INSTANTIABLE FINAL;
CREATE TYPE married UNDER client (
  partner_name VARCHAR2(400)
) INSTANTIABLE FINAL;
CREATE TABLE clients OF client(
  id CONSTRAINT clients__id__pk PRIMARY KEY
);
INSERT INTO clients VALUES ( SINGLE( 1, 'Alice', 'Adams', DATE '1900-01-01' ) )
1 rows affected

INSERT INTO clients VALUES ( MARRIED( 2, 'Betty', 'Baron', 'Bob' ) )
1 rows affected

ALTER SESSION SET NLS_DATE_FORMAT = 'YYYY-MM-DD HH24:MI:SS';
SELECT c.*,
       TREAT(VALUE(c) AS single).date_of_birth AS date_of_birth,
       TREAT(VALUE(c) AS married).partner_name AS partner_name
FROM   clients c


ID
FIRST_NAME
LAST_NAME
DATE_OF_BIRTH
PARTNER_NAME


1
Alice
Adams
1900-01-01 00:00:00
null

2
Betty
Baron
null
Bob