I've used them (in postgresql) to evaluate the merging of "parent" and "child" values - e.g. one row can be parent, and if the child has NULL for value it takes it from the parent (though do not remember much of the details). This can go to deeper just first parent, but parent of parent too.
I was able to make a query like this in PostgreSQL, but forgot the details of how I did it (though once you sit down, you get it eventually).
I was able to make a query like this in PostgreSQL, but forgot the details of how I did it (though once you sit down, you get it eventually).