select * from addresses left  join users on addresses.session_key = users.session_key where users.session_key is null;
 session_key |   address    | session_key | username
-------------+--------------+-------------+----------
       10009 | john_address |             |

# delete from addresses where session_key in (select session_key from addresses left join users on addresses.session_key = users.session_key where users.session_key is null);
ERROR:  column reference "session_key" is ambiguous
LINE 1: ...elete from addresses where session_key in (select session_ke...
                                                             ^
# delete from addresses where session_key in (select addr.session_key from addresses as addr left join users on addr.session_key = users.session_key where users.session_key is null);
DELETE 1
test=# select * from addresses left  join users on addresses.session_key = users.session_key where users.session_key is null;
 session_key | address | session_key | username
-------------+---------+-------------+----------
(0 rows)