text: --
  --    Copyright 2009-2016 the original author or authors.
  --
  --    Licensed under the Apache License, Version 2.0 (the "License");
  --    you may not use this file except in compliance with the License.
  --    You may obtain a copy of the License at
  --
  --       http://www.apache.org/licenses/LICENSE-2.0
  --
  --    Unless required by applicable law or agreed to in writing, software
  --    distributed under the License is distributed on an "AS IS" BASIS,
  --    WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  --    See the License for the specific language governing permissions and
  --    limitations under the License.
  --
  
  DROP TABLE parent IF EXISTS;
  DROP TABLE child IF EXISTS;
  DROP TABLE pet IF EXISTS;
  
  CREATE TABLE parent
  (
      id   INT,
      col1 VARCHAR(20),
      col2 VARCHAR(20)
  );
  
  CREATE TABLE child
  (
      id        INT,
      parent_id INT,
      name      VARCHAR(20)
  );
  
  CREATE TABLE pet
  (
      id        INT,
      parent_id INT,
      name      VARCHAR(20)
  );
  
  INSERT INTO parent (id, col1, col2)
  VALUES (1, NULL, NULL),
         (2, NULL, NULL);
  
  INSERT INTO child (id, parent_id, name)
  VALUES (1, 2, 'john'),
         (2, 2, 'mary');
  