There is actually a Wizard in Access that compares two tables to find missing information. When you go to New Query, you should be able to start the wizard. However, if you want to build this yourself, you would create a new query and bring in both tables. You will need to make a join relationship between the field in the Stock table with the field in the Production table that you want to compare against. Set the properties of the relationship as follows: Show all records from Stock and include only the records from Production that are matching (or whatever the exact wording is).
Now that you made the relationship between your two tables, you just set up your query to show both fields from both tables. If you run it right now, you will see that the field from the Stock table will show all the records of that field, and the Production field will only show matching records, otherwise it will be blank. Almost there....If you go back in to the design mode and enter IsNull in the Criteria field of the Production field and run the report again, you will see that your query now only shows the records from Stock where there are no matching fields in Production.
Again, the easiest thing is just to run through the "Unmatched" query wizard and then look at the query to see how the query is set up.
