Okay, slow down a bit. If you've never made a query based on more than one table it's not too difficult and you'll be amazed at what you can do afterward (ok, so I was amazed 6 months ago).
You're probably using the Query Wizard to build your queries, which is fine. For future reference, on that first (second?) screen where it asks what fields you want, you can add the fields you need from one table/query, then change the dropdown to a different table/query and add fields from that.
However if you add two tables that are unrelated, or that Access thinks are unrelated, you can end up with a very strange beast called a Cartesian product. What that means is that for EVERY row in table 1, it adds an entry for table 2. So if table 1 has 350 rows and table 2 has 100 rows, you'll end up with a query with 350x100 = 35,000 rows!
There are two ways to fix this and, as you guessed, they have to do with relationships.
1) From Tools>Relationships you can define permanent relationships between tables. For instance, linking your Primary Key from your main table to the Foreign Key slot that corresponds to it on your subtable. That relationship will always be true so you can define it permanently here and Enforce Referential Integrity, etc.
2) Go into your query in Design View and you'll see at the top a smaller version of the Relationships window, or at least it looks similar. If you've got permanent relationships between the tables in your query they will be shown, which can create quite a snarl sometimes. Stretching this portion of the design window and expanding your table views can clarify this if you need to.
In either case, you 'link fields' or 'create a relationship' between the two tables by dragging one of the matching fields over its counterpart in the other table. In the permanent view you'll be asked some options to confirm. If you need to change a relationship ever, click on it, right-click and go to Properties.
If you made a query with one table and want to add another, as I suspect you have here, right-click on the grey area at the top of your query's Design View. Show Table, and choose the table/query you want to add. Nothing simpler. You can double-click or drag down fields you want to add to the display, but you probably knew that.
See if you can get that working for your DOB table and your main table. Of course if this data is a field you need to add to your main table that's a slightly different type of query. It's never easy when you're getting your data piecemeal.
Good luck,
David R
