在5.5到5.6.4版本里，对于DEFAULT CURRENT_TIMESTAMP子句，只能TIMESTAMP类型列上指定。
而从5.6.5开始（也包括5.7），DEFAULT CURRENT_TIMESTAMP子句可以指定到TIMESTAMP或者DATETIME类型列上。
比如在5.5中：
mysql> select version();
+------------+
| version() |
+------------+
| 5.5.48-log |
+------------+
1 row in set (0.00 sec)
mysql> use test;
Database changed
mysql> create table t1(id int, dt timestamp not null DEFAULT CURRENT_TIMESTAMP, dt1 datetime not null DEFAULT CURRENT_TIMESTAMP);
ERROR 1067 (42000): Invalid default value for 'dt1'
mysql>
而在5.7中：
mysql> select version();
+------------+
| version() |
+------------+
| 5.7.11-log |
+------------+
1 row in set (0.00 sec)
mysql> use test;
Database changed
mysql> create table t1(id int, dt timestamp not null DEFAULT CURRENT_TIMESTAMP, dt1 datetime not null DEFAULT CURRENT_TIMESTAMP);
ERROR 1050 (42S01): Table 't1' already exists
mysql> drop table t1;
Query OK, 0 rows affected (0.38 sec)
mysql> create table t1(id int, dt timestamp not null DEFAULT CURRENT_TIMESTAMP, dt1 datetime not null DEFAULT CURRENT_TIMESTAMP);
Query OK, 0 rows affected (0.32 sec)
mysql> desc t1;
+-------+-----------+------+-----+-------------------+-------+
| Field | Type | Null | Key | Default | Extra |
+-------+-----------+------+-----+-------------------+-------+
| id | int(11) | YES | | NULL | |
| dt | timestamp | NO | | CURRENT_TIMESTAMP | |
| dt1 | datetime | NO | | CURRENT_TIMESTAMP | |
+-------+-----------+------+-----+-------------------+-------+
3 rows in set (0.10 sec)
mysql>