You are making it too convoluted. Since you are using a MySQL DB, here is the better solution:

-Keep a listing of all users in the DB... each player should correspond to one user and each user has a unique User ID (UID).
-When a client logs in, they log in as a user. Now you assign that UID to the Client's Pointer in the DB.
-When the client creates a house, it is assigned this UID.
-Now you trace through the DB: house --> UID... UID --> User... User---> Client Pointer.

There are more efficient ways of doing this in the DB (I'm no DB expert) but you get the idea... use a series of "lookup tables" to assign each object in the world to the right client and thus the right pointer and thus can send to the right person.