MySQL 在更新时自行更改 'changed' 列

Question

create table test (id int, changed timestamp);

insert into test (id, changed) values (1, now());
insert into test (id, changed) values (2, now());
insert into test (id, changed) values (3, now());

select * from test;

+----+---------------------+
| id | changed             |
+----+---------------------+
| 1  | 2015-07-14 14:58:00 | <-- correct
| 2  | 2015-07-14 14:58:02 |
| 3  | 2015-07-14 14:58:04 |
+----+---------------------+


update test set id = 5 where id = 1;  -- nothing with 'changed' specified!

select * from test;

+----+---------------------+
| id | changed             |
+----+---------------------+
| 5  | 2015-07-14 15:00:00 | <-- wrong! why?
| 2  | 2015-07-14 14:58:02 |
| 3  | 2015-07-14 14:58:04 |
+----+---------------------+

看到列已更改，我想设置 id 但未更改。数据库第一次对我执行此操作。

MySQL 为什么要这样做？

我怎样才能阻止这种行为？

Answer 1

默认情况下，TIMESTAMP 列将设置为 CURRENT_TIMESTAMP，并且每当您更新行时也会使用该列。

当您使用以下方式创建表格时:

CREATE TABLE test (
  id int,
  changed TIMESTAMP
);

它将与此相同:

CREATE TABLE test (
  id int,
  changed TIMESTAMP DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP
);

对于常量，默认值为给定值。在这种情况下，该列根本没有自动属性:

CREATE TABLE test (
  id int,
  changed TIMESTAMP 0
);