如何在 MySQL 中删除现有列上的 ON UPDATE CURRENT_TIMESTAMP?
ON UPDATE CURRENT_TIMESTAMP 定义了在没有显式时间戳的情况下更新将导致更新为当前时间戳值。
您可以使用 ALTER 命令从列中删除 ON UPDATE CURRENT_TIMESTAMP。
语法如下所示
ALTER TABLE yourTableName CHANGE yourTimeStampColumnName yourTimeStampColumnName timestamp NOT NULL default CURRENT_TIMESTAMP;
为了理解上述语法,让我们创建一个表。创建表的查询如下所示
mysql> create table removeOnUpdateCurrentTimeStampDemo - > ( - > Id int NOT NULL AUTO_INCREMENT PRIMARY KEY, - > Name varchar(20), - > UserUpdateTimestamp timestamp NOT NULL default CURRENT_TIMESTAMP on update CURRENT_TIMESTAMP - > ); Query OK, 0 rows affected (0.54 sec)
使用 DESC 命令检查表的描述。
查询如下所示
mysql> desc removeOnUpdateCurrentTimeStampDemo;
以下是输出
+---------------------+-------------+------+-----+-------------------+-----------------------------+ | Field | Type | Null | Key | Default | Extra | +---------------------+-------------+------+-----+-------------------+-----------------------------+ | Id | int(11) | NO | PRI | NULL | auto_increment | | Name | varchar(20) | YES | | NULL | | | UserUpdateTimestamp | timestamp | NO | | CURRENT_TIMESTAMP | on update CURRENT_TIMESTAMP | +---------------------+-------------+------+-----+-------------------+-----------------------------+ 3 rows in set (0.04 sec)
现在查看 Extra 字段,其中有 on update CURRENT_TIMESTAMP。删除 ON UPDATE CURRENT_TIMESTAMP 的查询如下所示
mysql> alter table removeOnUpdateCurrentTimeStampDemo - > change UserUpdateTimestamp UserUpdateTimestamp timestamp NOT NULL default CURRENT_TIMESTAMP; Query OK, 0 rows affected (0.16 sec) Records: 0 Duplicates: 0 Warnings: 0
再次检查表的描述。
查询如下所示
mysql> desc removeOnUpdateCurrentTimeStampDemo;
以下是输出
+---------------------+-------------+------+-----+-------------------+----------------+ | Field | Type | Null | Key | Default | Extra | +---------------------+-------------+------+-----+-------------------+----------------+ | Id | int(11) | NO | PRI | NULL | auto_increment | | Name | varchar(20) | YES | | NULL | | | UserUpdateTimestamp | timestamp | NO | | CURRENT_TIMESTAMP | | +---------------------+-------------+------+-----+-------------------+----------------+ 3 rows in set (0.00 sec)
如果要删除默认的 CURRENT_TIMESTAMP,则查询如下所示
mysql> alter table removeOnUpdateCurrentTimeStampDemo - > change UserUpdateTimestamp UserUpdateTimestamp timestamp NOT NULL; Query OK, 0 rows affected (0.12 sec) Records: 0 Duplicates: 0 Warnings: 0
再次检查表的描述。
查询如下所示
mysql> desc removeOnUpdateCurrentTimeStampDemo;
以下是输出
+---------------------+-------------+------+-----+---------+----------------+ | Field | Type | Null | Key | Default | Extra | +---------------------+-------------+------+-----+---------+----------------+ | Id | int(11) | NO | PRI | NULL | auto_increment | | Name | varchar(20) | YES | | NULL | | | UserUpdateTimestamp | timestamp | NO | | NULL | | +---------------------+-------------+------+-----+---------+----------------+ 3 rows in set (0.00 sec)
现在查看上面的示例输出,我们已经删除了 ON UPDATE CURRENT TIMESTAMP。
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