MySQL默认值使用另一列

Question

我有一张 table

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| id | date |
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现在我需要添加“周”列，那么是否可以添加类似的列

alter table `a` add column `week` INT(6) not null default DATE_FORMAT(`date`, '%Y%v')

？

Answer 1

上面关于使用触发器的评论是继续此操作的方法，但没有详细说明为什么不能这样做。所以，从 documentation ...

The DEFAULT value clause in a data type specification indicates 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 TIMESTAMP and DATETIME columns.