SQL Queries For An Access Database

Hi,

I'm working on an electronic register which people use to sign in and out of a building. It uses 3 tables, one to store personal information (Name & Visitor Number), one to store what time they come in, and one to store what time they leave. Visitor Number is the primary key in the Visitor_Info table, and is used as a foreign key in the other 2 tables.

I've been asked to put in an 'emergency print' function where, when you click the print button, it compares the data in the time in database and the time out database and prints a list of people still in the building.

So, what I'm trying to do is create a SQL query that will give me this information. Having never used SQL before, it's a little tricky. I've spent the last few days looking at tutorials on the net and learning about queries and this is what I've come up with:

Select Time_In_Table.Visitor_Number
Not Union
Select Time_Out_Table.Visitor_Number

If I'm right (and I'm probably not), this will give me the people who are in the time in table, but not in the time out table.

Can someone please help me by letting me know what I'm doing wrong in the statement above, and also, advise me as to where it goes since none of the tutorials I've found cover where to put the statement in my programming? At a guess, I'd say it goes in the print button click function.