Question

Say there are two relations r and s such that the foreign key B of r references the primary key A of s. How can the trigger mechanism be used to implement the on delete cascade option, when a tuple is deleted from s.

Answer 1

In SQLite syntax:

CREATE TRIGGER
  AFTER DELETE ON s
  FOR EACH ROW
BEGIN
  DELETE FROM r WHERE r.B = old.A;
END;

This creates a trigger that run on each row deleted from s.
The trigger deletes the corresponding records from r, given the foreign key relationship you indicated.