程序代码：

mysql> select * from threads;
+------+---------+
| tid  | subject |
+------+---------+
|    1 | A       |
|    2 | B       |
|    3 | C       |
+------+---------+
3 rows in set (0.00 sec)

程序代码：

mysql> select tid,subject from threads where tid in (1,2);
+------+---------+
| tid  | subject |
+------+---------+
|    1 | A       |
|    2 | B       |
+------+---------+
2 rows in set (0.00 sec)

mysql> select tid,subject from threads where tid in ();
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near ')' at line 1