mysql> SELECT p.id, p.PrincipalType, txn.id txn_id, txn.Field, txn.NewValue,
    -> txn.ReferenceType, txn.NewReference, t.id tid FROM Tickets t JOIN
    -> Transactions txn ON txn.Type = 'AddWatcher' AND txn.ObjectType =
    -> 'RT::Ticket' AND txn.ObjectId = t.id LEFT JOIN Principals p ON p.id =
    -> txn.NewValue WHERE p.id IS NULL AND t.id = 55767;
+------+---------------+--------+-------+----------+---------------+--------------+-------+
| id   | PrincipalType | txn_id | Field | NewValue | ReferenceType | NewReference | tid   |
+------+---------------+--------+-------+----------+---------------+--------------+-------+
| NULL | NULL          | 377862 | Cc    | 3368     | NULL          |         NULL | 55767 |
+------+---------------+--------+-------+----------+---------------+--------------+-------+
1 row in set (0.00 sec)