[Dovecot] password_query, user_query and unrelated userid and emails

Let's say one has a SQL table with following fields for John Doe:

login:		u0007
password:	{SHA1}...
mailhome:	/path/to/joeshome
email:		john.doe@example.com

To log in, for internal policy reasons, John MUST make use of his userid "u0007". So, in the password_query, one would need something like:

... WHERE login = '%u' ...

On the other hand, for receiving mail thru say, LMTP, one would need something like this in the user_query:

... WHERE email = '%Lu' ...

With such a simplistic setup, this works fine:

$ doveadm auth u0007
Password:
passdb: u0007 auth succeeded

but this one obviously fails:

$ doveadm user u0007
userdb lookup: user u0007 doesn't exist

and one has to do:

$ doveadm user john.doe@example.com
userdb: john.doe@example.com
  home       : /path/to/joeshome

I can't manage to devise a way to systematically refer to users thru their login, nor to avoid such messages:

prefetch(u0007): passdb didn't return userd entries, trying the next userdb

There sure must be a way thru adequate "SELECT ... AS ..." clauses. Could someone show me the light?

Many, many TIA, Axel