例子，mysql now函数与sleep函数。
SELECT NOW(), SLEEP(2), NOW();
sql语句如下：
mysql>
mysql> SELECT NOW(), SLEEP(2), NOW();
+---------------------+----------+---------------------+
| NOW() | SLEEP(2) | NOW() |
+---------------------+----------+---------------------+
| 2007-07-22 19:46:13 | 0 | 2007-07-22 19:46:13 |
+---------------------+----------+---------------------+
1 row in set (2.02 sec)
