MySQL中的系统函数sleep，实际应用的场景不多，一般用来做实验测试，昨天在测试的时候，意外发现sleep函数的一个特殊现象。如果在查询语句中使用sleep函数，那么休眠的时间跟返回的记录有关。如下测试所示：
mysql> create table test(id int);
Query OK, 0 rows affected (0.03 sec)
mysql> select *, sleep(6) from test;
Empty set (0.00 sec)
mysql> insert into test values(1);
Query OK, 1 row affected (0.00 sec)
mysql> select * ,sleep(6) from test;
+------+----------+
| id | sleep(6) |
+------+----------+
| 1 | 0 |
+------+----------+
1 row in set (6.00 sec)
mysql> insert into test value(2);
Query OK, 1 row affected (0.01 sec)
mysql> select * ,sleep(6) from test;
+------+----------+
| id | sleep(6) |
+------+----------+
| 1 | 0 |
| 2 | 0 |
+------+----------+
2 rows in set (12.00 sec)