If you are querying for user_id by definition the table contains user_id. I think what they are suggesting is checking that the data returned from the query has the correct user_id. No one would do that because if you can't trust your database to answer queries correctly you have more problems than just that. My guess with the Royal Mail is that it is probably something dumb like using a field in (Java) servlet to hold the data which is then shared across all sessions. I have seen that multiple times in my career. People don't notice when testing as there is only one session at a time.