title: SQL Error: ORA-02298: cannot validate (SYSTEM.AEROPUERTO_FK) - parent keys not found
body: <p>I'm getting the following error in on Oracle SQL Developer:</p>
  
  <pre><code>Error starting at line 1 in command:
      ALTER TABLE AEROPUERTO ADD CONSTRAINT AEROPUERTO_FK FOREIGN KEY (CODIGO_CIUDAD) REFERENCES CIUDAD(CODIGO)
  
  Error report:
      SQL Error: ORA-02298: cannot validate (SYSTEM.AEROPUERTO_FK) - parent keys not found
      02298. 00000 - "cannot validate (%s.%s) - parent keys not found"
      *Cause:    an alter table validating constraint failed because the table has
                 child records.
      *Action:   Obvious
  </code></pre>
  
  <p>Why?</p>