i go one step further and create a new database.. the mysql class isn't married to a particular database, so you can do cross DB joins with no code changes at all to the sql class..

Basically, select queries can look like:

SQL Query
SELECT my.FieldName, my.user_id, u.USER_ID
FROM $myDB.TableName my, {$config['TABLE_PREFIX']}USERS u
WHERE my.user_id=u.USER_ID

kinda stuff.. the only thing you have to make sure of is that the DB user and DB pass can connect to both DBs (duh) and the query is just as efficient as a intra table join within the same DB..

i stumbled upon it via google about 5yrs ago Linky Poo™