explicit_defaults_for_timestamp参数

Introduced	5.6.6
Deprecated	5.6.6
Command-Line Format	--explicit_defaults_for_timestamp=#
System Variable	Name	explicit_defaults_for_timestamp
	Variable Scope	Global, Session
	Dynamic Variable	No
Permitted Values	Type	boolean
	Default	FALSE

 在mysql中：
- timestamp列如果没有显式定义为null，默认会被设置为not null属性。(其它的数据类型如果没有显式定义为not null，默认是可以为null的)。设置timestamp的列值为null，会自动存储为当前timestamp
- 表中的第一个timestamp列，如果没有定义为null、定义default值或者on update，会自动分配default current_timestamp和on update current_timestamp属性
- 表中第一个timestamp列之后的所有timestamp列，如果没有被定义为null、定义default值，会自动被指定默认值'0000-00-00 00:00:00'。在插入时，如果没有指定这些列的值，会自动指定为'0000-00-00 00:00:00'，且不会产生警告

mysql> create table timestamp_eg(
    ->     id int not null auto_increment,
    ->     time1 timestamp,
    ->     time2 timestamp,
    ->     time3 timestamp NOT NULL DEFAULT '2010-01-01 00:00:00',
    ->     time4 timestamp,
    ->     primary key(id));
Query OK, 0 rows affected (0.01 sec)

mysql> insert into timestamp_eg(id) values(1);
Query OK, 1 row affected (0.00 sec)

mysql> select * from timestamp_eg;
+----+---------------------+---------------------+---------------------+---------------------+
| id | time1               | time2               | time3               | time4               |
+----+---------------------+---------------------+---------------------+---------------------+
|  1 | 2015-12-16 09:23:33 | 0000-00-00 00:00:00 | 2010-01-01 00:00:00 | 0000-00-00 00:00:00 |
+----+---------------------+---------------------+---------------------+---------------------+
1 row in set (0.01 sec)

mysql> update timestamp_eg set id=2 where id=1;
Query OK, 1 row affected (0.02 sec)
Rows matched: 1  Changed: 1  Warnings: 0

mysql> select * from timestamp_eg;
+----+---------------------+---------------------+---------------------+---------------------+
| id | time1               | time2               | time3               | time4               |
+----+---------------------+---------------------+---------------------+---------------------+
|  2 | 2015-12-16 09:25:01 | 0000-00-00 00:00:00 | 2010-01-01 00:00:00 | 0000-00-00 00:00:00 |
+----+---------------------+---------------------+---------------------+---------------------+
1 row in set (0.00 sec)

mysql> insert into timestamp_eg(id,time4) values(3,'2011-01-01 00:00:00');
Query OK, 1 row affected (0.00 sec)

mysql> select * from timestamp_eg;
+----+---------------------+---------------------+---------------------+---------------------+
| id | time1               | time2               | time3               | time4               |
+----+---------------------+---------------------+---------------------+---------------------+
|  2 | 2015-12-16 09:25:01 | 0000-00-00 00:00:00 | 2010-01-01 00:00:00 | 0000-00-00 00:00:00 |
|  3 | 2015-12-16 09:28:04 | 0000-00-00 00:00:00 | 2010-01-01 00:00:00 | 2011-01-01 00:00:00 |
+----+---------------------+---------------------+---------------------+---------------------+
2 rows in set (0.00 sec)

mysql> update timestamp_eg set id=4 where id=3;
Query OK, 1 row affected (0.00 sec)
Rows matched: 1  Changed: 1  Warnings: 0

mysql> select * from timestamp_eg;
+----+---------------------+---------------------+---------------------+---------------------+
| id | time1               | time2               | time3               | time4               |
+----+---------------------+---------------------+---------------------+---------------------+
|  2 | 2015-12-16 09:25:01 | 0000-00-00 00:00:00 | 2010-01-01 00:00:00 | 0000-00-00 00:00:00 |
|  4 | 2015-12-16 09:28:24 | 0000-00-00 00:00:00 | 2010-01-01 00:00:00 | 2011-01-01 00:00:00 |
+----+---------------------+---------------------+---------------------+---------------------+
2 rows in set (0.00 sec)

mysql> 

从MySQL5.6.6这种默认设置的方法被废弃了。在MySQL启动时会出现以下警告：

[Warning] TIMESTAMP with implicit DEFAULT value is deprecated.
Please use --explicit_defaults_for_timestamp server option (see
documentation for more details).

要想取消该警告，在启动mysql时，my.cnf中加入

[mysqld]
explicit_defaults_for_timestamp=true

修改该参数后，timestamp类型的列的处理方式也会发生变化：

- timestamp列如果没有显式定义为not null，则支持null属性。设置timestamp的列值为null，就不会被设置为current timestamp
- 不会自动分配default current_timestamp和on update current_timestamp属性，这些属性必须显式指定
- 声明为not null且没有显式指定默认值是没有默认值的。表中插入列，又没有给timestamp列赋值时，如果是严格sql模式，会抛出一个错误；如果严格sql模式没有启用，该列会赋值为’0000-00-00 00:00:00′，同时出现一个警告。（这和mysql处理其它时间类型数据一样，如datetime）

mysql> create table timestamp_02(
    -> id int not null auto_increment,
    -> time1 timestamp not null,
    -> primary key(id)
    -> );
Query OK, 0 rows affected (0.03 sec)

mysql> insert into timestamp_02(id) values(1);
ERROR 1364 (HY000): Field 'time1' doesn't have a default value
mysql>