mysql> select * from t2;

+----+---------------------+

| id | d                   |

+----+---------------------+

|  1 | 2015-08-02 15:13:59 |

|  3 | 2015-08-05 16:56:23 |

|  4 | 2015-08-05 16:56:24 |

|  6 | 2015-08-05 16:56:26 |

|  7 | 2015-08-05 16:56:28 |

|  8 | 2015-08-05 16:56:29 |

+----+---------------------+

6 rows in set (0.00 sec)

mysql> set @x=0;

Query OK, 0 rows affected (0.00 sec)

mysql> select @x:=ifnull(@x,0)+1 as rownum,d as time from t2;

+--------+---------------------+

| rownum | time                |

+--------+---------------------+

|      1 | 2015-08-02 15:13:59 |

|      2 | 2015-08-05 16:56:23 |

|      3 | 2015-08-05 16:56:24 |

|      4 | 2015-08-05 16:56:26 |

|      5 | 2015-08-05 16:56:28 |

|      6 | 2015-08-05 16:56:29 |

+--------+---------------------+

6 rows in set (0.00 sec)