仅在插入新行时设置 current_timestamp

Question

列 date 是 timestamp - 默认值 - CURRENT_TIMESTAMP

我选择这个是因为想要在 inserting 新行上显示当前日期时间。

但是每次更新另一列时它都会改变。

有没有办法让 current_timestamp 仅在插入新行时 而不是 在更新另一列时？

Answer 1

An auto-updated column is automatically updated to the current timestamp when the value of any other column in the row is changed from its current value. An auto-updated column remains unchanged if all other columns are set to their current values. To prevent an auto-updated column from updating when other columns change, explicitly set it to its current value. To update an auto-updated column even when other columns do not change, explicitly set it to the value it should have (for example, set it to CURRENT_TIMESTAMP).

因此，您要避免 my_date_col 列在 UPDATE 时自动更新，您希望明确保留其值，例如：

UPDATE my_table SET my_col1 = 'foo', my_col2 = 'bar', my_date_col = my_date_col
WHERE ...

Answer 2

TIMESTAMP 列在每次更新时自动更新。 DATETIME 列没有。

你想要：

your_column DATETIME DEFAULT CURRENT_TIMESTAMP

这让您可以在插入时指定一个默认值，就像您对任何列所做的那样，但是因为它不再是 TIMESTAMP 它不会继续在每个未来更新自己 UPDATE.

Answer 3

当你运行

SHOW FULL COLUMNS FROM `moufa`; -- where `moufa` is the name of the table in the example

你会看到这样的东西

+-------+-------------+-----------+------+-----+---------------------+-------------------------------+---------------------------------+---------+
| Field | Type        | Collation | Null | Key | Default             | Extra                         | Privileges                      | Comment |
+-------+-------------+-----------+------+-----+---------------------+-------------------------------+---------------------------------+---------+
| id    | smallint(6) | NULL      | NO   | PRI | NULL                | auto_increment                | select,insert,update,references |         |
| ts    | timestamp   | NULL      | NO   |     | current_timestamp() | on update current_timestamp() | select,insert,update,references |         |
+-------+-------------+-----------+------+-----+---------------------+-------------------------------+---------------------------------+---------+

问题出在 Extra 的 ts 字段上。检查你的 table 运行

SHOW CREATE TABLE `moufa`;

像GMB这样的最常见案例已自动更新专栏。

现在为了避免更多类似的情况，您应该明确定义列的默认值。

DROP TABLE IF EXISTS `moufa`; -- just for the example
CREATE TABLE `moufa`( 
    `id` SMALLINT NOT NULL AUTO_INCREMENT, 
    `ts` TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
    PRIMARY KEY(`id`)
);

或者不是删除并重新创建 table

ALTER TABLE `moufa`
  MODIFY COLUMN `ts` TIMESTAMP DEFAULT CURRENT_TIMESTAMP;

仅在插入新行时设置 current_timestamp

set current_timestamp only on inserting a new row

mysql

date

create-table

sql-update

sql-insert