CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP ：当我更新这条记录的时候，这条记录的这个字段将会改变。即时间变为了更新时候的时间。（注意一个UPDATE设置一个列为它已经有的值，这将不引起TIMESTAMP列被更新，因为如果你设置一个列为它当前的值，MySQL为了效率而忽略更改。）

如果有多个TIMESTAMP列，只有第一个自动更新。

#1创建一个有两个timestamp字段的表dj1。
root@localhost:test >create table dj1 (a char(1), b timestamp ,c timestamp);
Query OK, 0 rows affected (0.01 sec)

#2插入两行数据，仅赋值于列A
root@localhost:test >insert into dj1 values (1,null,null);
Query OK, 1 row affected (0.00 sec)

root@localhost:test >insert into dj1 values (2,null,null);
Query OK, 1 row affected (0.00 sec)

#3查询插入数据，b,c两列都使用current timestamp作为默认值。
root@localhost:test >select * from dj1;
+------+---------------------+---------------------+
| a | b | c |
+------+---------------------+---------------------+
| 1 | 2009-09-09 13:48:40 | 2009-09-09 13:48:40 |
| 2 | 2009-09-09 13:48:44 | 2009-09-09 13:48:44 |
+------+---------------------+---------------------+
2 rows in set (0.00 sec)

#4更新一行数据，发现b列timestamp被自动更新，而c列保持不变。
root@localhost:test >update dj1 set a=9 where a=1;
Query OK, 1 row affected (0.00 sec)
Rows matched: 1 Changed: 1 Warnings: 0

root@localhost:test >select * from dj1;
+------+---------------------+---------------------+
| a | b | c |
+------+---------------------+---------------------+
| 9 | 2009-09-09 13:49:08 | 2009-09-09 13:48:40 |
| 2 | 2009-09-09 13:48:44 | 2009-09-09 13:48:44 |
+------+---------------------+---------------------+
2 rows in set (0.00 sec)

#5再更新一列，仍然如#4
root@localhost:test >update dj1 set a=8 where a=2;
Query OK, 1 row affected (0.00 sec)
Rows matched: 1 Changed: 1 Warnings: 0

root@localhost:test >select * from dj1;
+------+---------------------+---------------------+
| a | b | c |
+------+---------------------+---------------------+
| 9 | 2009-09-09 13:49:08 | 2009-09-09 13:48:40 |
| 8 | 2009-09-09 13:49:36 | 2009-09-09 13:48:44 |
+------+---------------------+---------------------+
2 rows in set (0.00 sec)

#6在b列上创建唯一索引
root@localhost:test >create unique index dj1_idx_u1 on dj1(b);
Query OK, 2 rows affected (0.01 sec)
Records: 2 Duplicates: 0 Warnings: 0

#7更新所有行a列，报唯一性冲突。
root@localhost:test >update dj1 set a=1;
ERROR 1062 (23000): Duplicate entry '2009-09-09 13:54:45' for key 'dj1_idx_u1'

#8查看表定义，可以看到b列有个属性ON UPDATE CURRENT_TIMESTAMP，导致更新数据时，即便未涉及到该列，该列数据也被自动更新。
另一方面，c列默认值是'0000-00-00 00:00:00'，实际插入已经被自动赋值为current_timestamp。
root@localhost:test >show create table dj1\G
*************************** 1. row ***************************
Table: dj1
Create Table: CREATE TABLE `dj1` (
`a` char(1) COLLATE utf8_bin DEFAULT NULL,
`b` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
`c` timestamp NOT NULL DEFAULT '0000-00-00 00:00:00',
UNIQUE KEY `dj1_idx_u1` (`b`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_bin
1 row in set (0.00 sec)

#9创建表dj2，列b不带自动更新属性。
root@localhost:test >CREATE TABLE `dj2` (
-> `a` char(1) COLLATE utf8_bin DEFAULT NULL,
-> `b` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ,
-> `c` timestamp NOT NULL DEFAULT '0000-00-00 00:00:00',
-> UNIQUE KEY `dj1_idx_u1` (`b`)
-> ) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_bin;
Query OK, 0 rows affected (0.01 sec)

#10插入dj2测试数据
root@localhost:test >insert into dj2 values (1,null,null);
Query OK, 1 row affected (0.00 sec)

root@localhost:test >insert into dj2 values (2,null,null);
Query OK, 1 row affected (0.00 sec)

#11查看dj2数据
root@localhost:test >select * from dj2;
+------+---------------------+---------------------+
| a | b | c |
+------+---------------------+---------------------+
| 1 | 2009-09-09 14:02:55 | 2009-09-09 14:02:55 |
| 2 | 2009-09-09 14:03:00 | 2009-09-09 14:03:00 |
+------+---------------------+---------------------+
2 rows in set (0.00 sec)

#12dj2上创建唯一索引
root@localhost:test >create unique index dj2_idx_u1 on dj2(b);
Query OK, 2 rows affected (0.02 sec)
Records: 2 Duplicates: 0 Warnings: 0

#更新数据成功
root@localhost:test >update dj2 set a=9;
Query OK, 2 rows affected (0.00 sec)
Rows matched: 2 Changed: 2 Warnings: 0

root@localhost:test >select * from dj2;
+------+---------------------+---------------------+
| a | b | c |
+------+---------------------+---------------------+
| 9 | 2009-09-09 14:02:55 | 2009-09-09 14:02:55 |
| 9 | 2009-09-09 14:03:00 | 2009-09-09 14:03:00 |
+------+---------------------+---------------------+
2 rows in set (0.00 sec)

#13创建表dj3，b列默认值为CURRENT_TIMESTAMP，c列默认值为CURRENT_TIMESTAMP带自动更新属性，报错，不允许行为。
root@localhost:test >CREATE TABLE `dj3` (
-> `a` char(1) COLLATE utf8_bin DEFAULT NULL,
-> `b` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ,
-> `c` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
-> UNIQUE KEY `dj1_idx_u1` (`b`)
-> ) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_bin;
ERROR 1293 (HY000): Incorrect table definition; there can be only one TIMESTAMP column with CURRENT_TIMESTAMP in DEFAULT or ON UPDATE clause

#14创建表dj4，b列默认值为CURRENT_TIMESTAMP，c列默认值为'0000-00-00 00:00:00'带自动更新属性，报错，不允许行为。
root@localhost:test >CREATE TABLE `dj4` (
-> `a` char(1) COLLATE utf8_bin DEFAULT NULL,
-> `b` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ,
-> `c` timestamp NOT NULL DEFAULT '0000-00-00 00:00:00' ON UPDATE CURRENT_TIMESTAMP,
-> UNIQUE KEY `dj1_idx_u1` (`b`)
-> ) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_bin;
ERROR 1293 (HY000): Incorrect table definition; there can be only one TIMESTAMP column with CURRENT_TIMESTAMP in DEFAULT or ON UPDATE clause

#15创建表dj5，b列默认值为CURRENT_TIMESTAMP带自动更新属性，c列默认值为CURRENT_TIMESTAMP，报错，不允许行为。
root@localhost:test >CREATE TABLE `dj5` (
-> `a` char(1) COLLATE utf8_bin DEFAULT NULL,
-> `b` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
-> `c` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ,
-> UNIQUE KEY `dj1_idx_u1` (`b`)
-> ) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_bin;
ERROR 1293 (HY000): Incorrect table definition; there can be only one TIMESTAMP column with CURRENT_TIMESTAMP in DEFAULT or ON UPDATE clause

conclusion：
1.MySQL默认表的第一个timestamp字段为NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP属性，必须显式定义改变这种行为。
2.MySQL只允许一个timestamp字段拥有[DEFAULT CURRENT_TIMESTAMP |ON UPDATE CURRENT_TIMESTAMP]属性。 我的理解为要么都是DEFAULT CURRENT_TIMESTAMP 要么都是DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP

3.修改字段属性值