title: Simply adding a foreign key SQL
body: <p>I'm trying to add a foreign key constraint to my user_info table, that is a primary key in my users table (user_id):</p>
  
  <pre><code>ALTER TABLE user_info
  ADD CONSTRAINT fk_userID
  FOREIGN KEY (user_id)
  REFERENCES users(user_id);
  </code></pre>
  
  <p>However, I'm getting this error:</p>
  
  <pre><code>Error starting at line 26 in command:
  ALTER TABLE user_info
  ADD CONSTRAINT fk_userID
  FOREIGN KEY (user_id)
  REFERENCES users(user_id)
  Error report:
  SQL Error: ORA-02298: cannot validate (CLAYBANKS.FK_USERID) - 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>Any ideas?</p>