SELECT title 
FROM B
INNER JOIN (SELECT DISTINCT tid FROM A WHERE A.authorid=41) tmp 
ON B.tid=tmp.tid 
ORDER BY B.lasttime DESC

------解决方案--------------------
SQL code
mysql> select * from a;
+------+----------+
| tid  | authorid |
+------+----------+
|    1 |       41 |
|    2 |       41 |
|    3 |       41 |
|    3 |       41 |
|    4 |       55 |
+------+----------+
5 rows in set (0.00 sec)

mysql> select * from b;
+------+---------+----------+
| tid  | subject | lastpost |
+------+---------+----------+
|    1 | aa      |       11 |
|    2 | bb      |       22 |
|    3 | cc      |       33 |
|    4 | dd      |       44 |
|    5 | ee      |       55 |
+------+---------+----------+
5 rows in set (0.00 sec)

mysql> select * from b
    -> where exists (select 1 from a where tid=b.tid and authorid=41)
    -> order by lastpost desc
    -> ;
+------+---------+----------+
| tid  | subject | lastpost |
+------+---------+----------+
|    3 | cc      |       33 |
|    2 | bb      |       22 |
|    1 | aa      |       11 |
+------+---------+----------+
3 rows in set (0.00 sec)

mysql>

------解决方案--------------------
或者

SQL code

mysql> select * from b
    -> where tid in  (select tid from a where authorid=41)
    -> order by lastpost desc;
+------+---------+----------+
| tid  | subject | lastpost |
+------+---------+----------+
|    3 | cc      |       33 |
|    2 | bb      |       22 |
|    1 | aa      |       11 |
+------+---------+----------+
3 rows in set (0.00 sec)

mysql>