I am developing an inventory system to track the rail cars for my model train club. I have a table called ‘roster’ that has all of the cars in it. The primary field is road # which is unique to each car. I have an app that allows users to enter a road # and it brings up the details of the car using via the parameter ID. It works great. Now I want to add a second table that will track cars that are being repaired. The primary field is road name (which is not unique & could have many of the same names). I have the two tables linked via road #. I would like the app I have to somehow ‘check’ the repair table to see if that car road # is being repaired. parameter ID won’t work because they are in two tables … how can I link the two tables so that repair details are shown if the car is being repaired?
@frederikdc could you please help with this request? 
The Rosters and Repairs table should be linked with a Link to table field. You can add a Link to table field to the Repairs table and link it the Rosters table.
The Repairs table should also contain a boolean field indicating that the reparation is finished or not.
Once you have that information, you can create a data source in your application (the page where you show the details of a single car) that shows all the repairs where the car is the link to table field and the status is not finished.
Show this data source in a repeat element. It will be empty if no repair is active and show the details of the repairs that are not finished.