title: Foreign key without correspondence in the referenced table
body: <p>I'm trying to come up with a solution for an exercise in which I have to create 3 tables: one for employees, one for projects, and one for projects and employees, in which I have to insert the employee's ID and associate with a project ID. In the project_employee table, since it has only 2 columns that reference other tables, I thought I could set them bot as foreign keys, like this:</p>
  <pre><code>CREATE TABLE employee_project
  (
      id_employee numeric FOREIGN KEY REFERENCES employee(id_employee),
      id_project numeric FOREIGN KEY REFERENCES project(id_project)
  )
  </code></pre>
  <p>Then I stumbled upon a problem: when inserting values on the 3 tables, I noticed that one of the employee's ID number was 4, but on the table employee there was no line with ID 4. Of course, this line wasn't created, but I want to understand: is there a way I could create a line whose ID has no matching record in the referenced table? Could it be a possible mistake in the question, or is there something I'm missing? Thanks in advance for your time!</p>