由于MySQL目前字段的默认值不支持函数
所以以代码如下:
create_time datetime default now()
的形式设置默认值是不可能的。
代替的方案是使用TIMESTAMP类型代替DATETIME类型。
CURRENT_TIMESTAMP :当我更新这条记录的时候,这条记录的这个字段不会改变。
CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP :当我更新这条记录的时候,这条记录的这个字段将会改变。即时间变为了更新时候的时间。(注意一个UPDATE设置一个列为它已经有的值,这将不引起TIMESTAMP列被更新,因为如果你设置一个列为它当前的值,MySQL为了效率而忽略更改。)如果有多个TIMESTAMP列,只有第一个自动更新。
TIMESTAMP列类型自动地用当前的日期和时间标记INSERT或UPDATE的操作。
如果有多个TIMESTAMP列,只有第一个自动更新。
自动更新第一个TIMESTAMP列在下列任何条件下发生:
列值没有明确地在一个INSERT或LOAD DATA INFILE语句中指定。
列值没有明确地在一个UPDATE语句中指定且另外一些的列改变值。(注意一个UPDATE设置一个列为它已经有的值,这将不引起TIMESTAMP列被更新,因为如果你设置一个列为它当前的值,MySQL为了效率而忽略更改。)
你明确地设定TIMESTAMP列为NULL.
除第一个以外的TIMESTAMP列也可以设置到当前的日期和时间,只要将列设为NULL,或NOW()。
另外在5.0以上版本中也可以使用trigger来实现此功能。
代码如下:
create table test_time (
id int(11),
create_time datetime
);
delimiter |
create trigger default_datetime before insert on test_time
for each row
if new.create_time is null then
set new.create_time = now();
end if;|
delimiter ;

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