mysql> select * from groups_announcements;
+----------+-----------------+
| group_id | announcement_id |
+----------+-----------------+
|        4 |              12 |
|        4 |              13 |
|        5 |              14 |
|        5 |              15 |
+----------+-----------------+
4 rows in set (0.00 sec)

mysql> select announcement_id, time_posted from announcements;
+-----------------+---------------------+
| announcement_id | time_posted         |
+-----------------+---------------------+
|              11 | 2006-06-18 14:22:05 |
|              12 | 2006-07-13 10:07:57 |
|              13 | 2006-07-13 10:11:04 |
|              14 | 2006-07-13 12:19:12 |
|              15 | 2006-07-13 12:21:01 |
+-----------------+---------------------+
5 rows in set (0.00 sec)

mysql> select group_id, groups_announcements.announcement_id, time_posted from announcements left join groups_announcements using (announcement_id);
+----------+-----------------+---------------------+
| group_id | announcement_id | time_posted         |
+----------+-----------------+---------------------+
|     NULL |            NULL | 2006-06-18 14:22:05 |
|        4 |              12 | 2006-07-13 10:07:57 |
|        4 |              13 | 2006-07-13 10:11:04 |
|        5 |              14 | 2006-07-13 12:19:12 |
|        5 |              15 | 2006-07-13 12:21:01 |
+----------+-----------------+---------------------+
5 rows in set (0.00 sec)

select group_id, groups_announcements.announcement_id, time_posted from announcements as A1 left join groups_announcements using (announcement_id) where time_posted = (select max(A2.time_posted) from announcements AS A2 where A1.announcement_id = A2.announcement_id);
+----------+-----------------+---------------------+
| group_id | announcement_id | time_posted         |
+----------+-----------------+---------------------+
|     NULL |            NULL | 2006-06-18 14:22:05 |
|        4 |              12 | 2006-07-13 10:07:57 |
|        4 |              13 | 2006-07-13 10:11:04 |
|        5 |              14 | 2006-07-13 12:19:12 |
|        5 |              15 | 2006-07-13 12:21:01 |
+----------+-----------------+---------------------+
5 rows in set (0.00 sec)