MySQL error when including a file

[imported_ChrisF79]

I am using a php include to get the left navbar of my site to use in my forum. The problem is, whenever I do this, I get an error from the forum saying:

Invalid SQL:
SELECT
user.username, (user.options & 512) AS invisible, user.usergroupid,
session.userid, session.inforum, session.lastactivity,
IF(displaygroupid=0, user.usergroupid, displaygroupid) AS displaygroupid
FROM session AS session
LEFT JOIN user AS user ON(user.userid = session.userid)
WHERE session.lastactivity > 1101222104
ORDER BY username ASC

mysql error: Table 'understa_main.session' doesn't exist

Now, in my leftnav there is a query to a different database. Could it be that I need to close the connection to that database in the leftnav file? If so, how do I close a connection once I've queried the database?