[Old Pedant]

Well, I still haven't come up with a way to show you all users and all items.

But you *can* get a report on all itemid's for a single user.

I added some more data to my test table and then ran the query you will see below.

mysql> select itemid, userid, dateAdded from itemComments order by itemid, dateAdded;
+--------+--------+---------------------+
| itemid | userid | dateAdded           |
+--------+--------+---------------------+
| 383    | ABC4EF | 2012-11-16 23:39:22 |
| 383    | 1      | 2012-12-08 09:46:41 |
| 383    | 100    | 2012-12-08 09:47:48 |
| 383    | ABC4EF | 2012-12-09 00:00:00 |
| 383    | 1      | 2012-12-10 00:00:00 |
| 383    | 33     | 2012-12-13 00:00:00 |
| 383    | 44     | 2012-12-13 00:00:00 |
| 777    | 1      | 2011-01-01 00:00:00 |
| 777    | ABC4EF | 2012-01-01 00:00:00 |
| 777    | 2      | 2013-01-01 00:00:00 |
+--------+--------+---------------------+
10 rows in set (0.00 sec)

mysql> select C.itemid, COUNT(*) FROM itemcomments AS C,
    ->   (SELECT itemid, Max(dateadded) AS lastcomment FROM itemcomments where userid = 'ABC4EF' group by itemid ) AS M
    -> where C.itemid = M.itemid AND C.userid <> 'ABC4EF' AND C.dateadded > M.lastcomment
    -> group by C.itemid;
+--------+----------+
| itemid | COUNT(*) |
+--------+----------+
| 383    |        3 |
| 777    |        1 |
+--------+----------+
2 rows in set (0.02 sec)

It's correct. For userid ABC4EF there are indeed 3 comments after his last comment on itemid 383 but only 1 after his last comment on itemid 777.

Is this good enough? Or do you really need it for all userid's and all itemid's??