courses.uuid, units.uuid FROM units LEFT JOIN courses ON units.course_uuid = courses.uuid AND units.position > 2;

    uuid     |   uuid
-------------+-----------
 ruby-basics | RUBY-007
 ruby-basics | RUBY-006
 ruby-basics | RUBY-005
 ruby-basics | RUBY-END
 ruby-basics | RUBY-004
 ruby-basics | RUBY-003
 ruby-basics | RUBY-002
 ruby-basics | RUBY-001c
             | RUBY-001b
             | RUBY-001a
             | RUBY-000
(11 rows)



SELECT courses.uuid, units.uuid FROM units LEFT JOIN courses ON units.course_uuid = courses.uuid WHERE units.position > 2;

    uuid     |   uuid
-------------+-----------
 ruby-basics | RUBY-007
 ruby-basics | RUBY-006
 ruby-basics | RUBY-005
 ruby-basics | RUBY-END
 ruby-basics | RUBY-004
 ruby-basics | RUBY-003
 ruby-basics | RUBY-002
 ruby-basics | RUBY-001c
(8 rows)