How do I "link" two or more rows of the same table with a unique "link" id?
If the ID in your 'link' table absolutely has to be unique, and if it's ALWAYS a link between two IDs, you could use 3 columns: ID, min_position, max_position. That would allow you to use joins with your positions table.

I'm a bit confused regarding the goal of your application, i must admit ;).