Graphs have arbitrary depth. As a result some queries need an unknown number of joins on an unknown number of keys. Representing this in just SQL would be impos

Graphs have arbitrary depth. As a result some queries need an unknown number of joins on an unknown number of keys. Representing this in just SQL would be impossible. You could do it in a stored procedure but that is a superset of SQL not vanilla SQL. You can also do it by just making the SQL calls over and over in your code as you get each row but that's not going to be very optimized.