函数last_insert_id返回0

Question

我有承诺：

 mysql> CREATE TABLE auto(id int AUTO_INCREMENT PRIMARY KEY,num int);
    Query OK, 0 rows affected (1.40 sec)

    mysql> INSERT INTO auto VALUES(1,1);
    Query OK, 1 row affected (0.51 sec)

    mysql> SELECT LAST_INSERT_ID() FROM auto;
    +------------------+
    | LAST_INSERT_ID() |
    +------------------+
    |                0 |

为什么返回0？