title: column "parent_id" referenced in foreign key constraint does not exist when creating SQL table
body: <p>I am new in SQL and trying to understand Foreign key syntax. I know this was asked in multiple questions but each question I found did not seem to teach me what I am doing wrong here.
  This is my SQL code:</p>
  
  <pre><code>CREATE TABLE Customer
  (
  id int primary key,
  name varchar(30),
  age int,
  gender bool
  );
  
  CREATE TABLE Minor
  (
  FOREIGN KEY (parent_id) REFERENCES Customer(id)
  );
  
  CREATE TABLE Adult
  (
  FOREIGN KEY (parent_id) REFERENCES Customer(id)
  );
  
  CREATE TABLE Shop
  (
  id int primary key
  );
  
  CREATE TABLE Drink
  (
  name varchar(30) primary key
  );
  
  CREATE TABLE AlcoholicDrink
  (
  FOREIGN KEY (name) REFERENCES Drink(name)
  );
  
  CREATE TABLE NonAlcoholicDrink
  (
  FOREIGN KEY (name) REFERENCES Drink(name)
  );
  </code></pre>
  
  <p>And this is the error I am getting:</p>
  
  <pre><code>ERROR:  column "parent_id" referenced in foreign key constraint does not exist
  SQL state: 42703
  </code></pre>