I'd like to use the “nested set model” but I'm obliged to have a GUID as primary key. How can I do without integers as pk?

It doesn't matter what type your primary key is as the left/right values will still be integers. Eg.

As @Macka writes, the left and right values are not foreign keys to tree nodes, so they don't have to be the same type. They can be integers while the tree node primary key is a GUID.

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