MySQL将默认值设置为项目自动增量键

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只需做一个需要多个项目由AJAX重新排序的站点.

Just doing a site which requires multiple items to be reordered by AJAX.

当我在数据库中创建给定对象时，我想在创建时将position列设置为行的主键的值.

When I create a given object in the database, I want to set the position column to the value of the rows' primary key upon creation.

请问我该怎么做?

推荐答案

MySQL文档:

DEFAULT子句为列指定默认值.除一个例外，默认值必须为常数；否则为0.它不能是函数或表达式.例如，这意味着您不能将日期列的默认值设置为诸如NOW()或CURRENT_DATE之类的函数的值.唯一的例外是，您可以将CURRENT_TIMESTAMP指定为TIMESTAMP列的默认值.请参见第10.3.1.1节"TIMESTAMP属性".

The DEFAULT clause specifies a default value for a column. With one exception, the default value must be a constant; it cannot be a function or an expression. This means, for example, that you cannot set the default for a date column to be the value of a function such as NOW() or CURRENT_DATE. The exception is that you can specify CURRENT_TIMESTAMP as the default for a TIMESTAMP column. See Section 10.3.1.1, "TIMESTAMP Properties".

虽然您可以通过选择获得id值:

You can though obtain the id value on selection instead:

SELECT id, COALESCE(position, id) as position ...