[Dovecot] Tricky Master User Query - need MySQL help

[Marc Perkel]

  OK - I want to do something very tricky with master users and if I get 
it to work I'm adding it to the wiki for everyone to use.

So far I have a MySQL database that servers multiple domains. I can set 
a flag making someone a domain owner that allows that person to become 
master of that domain. Here's the query for that.

password_query = SELECT user_name, domain_name, password FROM users 
WHERE user_name = '%n' AND domain_name = '%d' AND owns_domain='1'  AND 
'%d'='%{login_domain}'

So - here's where we get tricky.

I have another table called "ownership" where I have 2 fields which is a 
pairing of email addresses (login_id) and domains owned (owned_object). 
Data might look like this:

joe at blow.com domain1.com
joe at blow.com domain2.com

Since joe at blow.com owns both domain1.com and domain2.com I want him to 
be able to log in as anyone in those 2 domains as a master user.

Examples:
   tom at domain1.com*joe at blow.com
   harry at domain2.com*joe at blow.com

So the idea is to create a MySQL query so that if the master user 
matches the domain part of the login user then it returns true and 
allows the login.

What MySQL query will do that?