|
mysql 中有这样的一个默认行为，如果一行数据中某些列被更新了，如果这一行中有timestamp类型的列，那么么这个timestamp列的数据
也会被自动更新到 更新操作所发生的那个时间点；这个操作是由explicit_defaults_for_timestamp这个变更控制的
一、体验一下mysql的默认更新行为：
mysql> create table t(x int ,y timestamp); -- 创建一个带有timestamp列的表
Query OK, 0 rows affected (0.01 sec)
mysql> insert into t(x) values(1); -- 只插x列
Query OK, 1 row affected (0.00 sec)
mysql> select * from t; -- timestamp列会自动更新
+------+---------------------+
| x | y |
+------+---------------------+
| 1 | 2017-06-07 13:48:56 |
+------+---------------------+
1 row in set (0.00 sec)
mysql> update t set x=2 where x=1; -- update 时timestamp列还是会自动更新
Query OK, 1 row affected (0.01 sec)
Rows matched: 1 Changed: 1 Warnings: 0
mysql> select * from t;
+------+---------------------+
| x | y |
+------+---------------------+
| 2 | 2017-06-07 13:49:21 |
+------+---------------------+
1 row in set (0.00 sec)
二、如何关闭这一默认行为：
聪明的你一想到了，只要把explicit_defaults_for_timestamp这个变更设置为on；对于timestamp列的值都要显示指定，那么这一默认行为就
算是关闭了。来操作一把！
mysql> set @@global.explicit_defaults_for_timestamp=on; -- 把全局的设置为on 那么新的连接就会被设置成on
Query OK, 0 rows affected (0.00 sec)
mysql> set @@session.explicit_defaults_for_timestamp=on; -- 把当前连接explicit_defaults_for_timestamp设置为on
Query OK, 0 rows affected (0.00 sec)
mysql> update t set x=3 ; -- 第一次更新 结果是变了
Query OK, 1 row affected (0.00 sec)
Rows matched: 1 Changed: 1 Warnings: 0
mysql> select * from t;
+------+---------------------+
| x | y |
+------+---------------------+
| 3 | 2017-06-07 13:57:21 |
+------+---------------------+
1 row in set (0.00 sec)
mysql> update t set x=4 ; -- 第二次更新 结果还是变了
Query OK, 1 row affected (0.00 sec)
Rows matched: 1 Changed: 1 Warnings: 0
mysql> select * from t;
+------+---------------------+
| x | y |
+------+---------------------+
| 4 | 2017-06-07 13:57:52 |
+------+---------------------+
1 row in set (0.00 sec)
mysql> show variables like 'explicit_defaults_for_timestamp'; -- 全局发动生效
+---------------------------------+-------+
| Variable_name | Value |
+---------------------------------+-------+
| explicit_defaults_for_timestamp | ON |
+---------------------------------+-------+
1 row in set (0.00 sec)
mysql> show global variables like 'explicit_defaults_for_timestamp'; -- session 级别改动也升效了
+---------------------------------+-------+
| Variable_name | Value |
+---------------------------------+-------+
| explicit_defaults_for_timestamp | ON |
+---------------------------------+-------+
1 row in set (0.00 sec)
-- 发现了吧、就算把explicit_defaults_for_timestamp 设置成了on 也不会有效的
三、找问题出在了哪里：
1、explicit_defaults_for_timestamp=off 时表结构
CREATE TABLE `t` (
`x` int(11) DEFAULT NULL,
`y` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
-- `y` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE
2、explicit_defaults_for_timestamp=on 时表结构
CREATE TABLE `t6` (
`x` int(11) DEFAULT NULL,
`y` timestamp NULL DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
-- `y` timestamp NULL DEFAULT NULL
四、结论：
explicit_defaults_for_timestamp 变量会直接影响表结构，也就是说explicit_defaults_for_timestamp的作用时间
是在表定义的时候；你的update | insert 想通过它去改变行为已经太晚了！
五、如何解决这失控的场面：
1、改表结构
mysql> alter table t modify column y timestamp null default null; -- 解决办法改表结构
Query OK, 0 rows affected (0.05 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> update t set x=1;
Query OK, 1 row affected (0.01 sec)
Rows matched: 1 Changed: 1 Warnings: 0
mysql> select * from t;
+------+---------------------+
| x | y |
+------+---------------------+
| 1 | 2017-06-07 13:59:21 |
+------+---------------------+
1 row in set (0.00 sec)
mysql> update t set x=2; -- 再次对比，发现时间没有变哦！
Query OK, 1 row affected (0.01 sec)
Rows matched: 1 Changed: 1 Warnings: 0
mysql> select * from t;
+------+---------------------+
| x | y |
+------+---------------------+
| 2 | 2017-06-07 13:59:21 |
+------+---------------------+
1 row in set (0.00 sec)
---------------------
(责任编辑：IT) |