问题描述

我需要表中的两列在插入时具有相同的值.有没有办法从数据库端做到这一点?

I need two columns in table that would have same value on insert. Is there any way to do it from database side?

推荐答案

所以你想让一个列使用 auto_increment 功能，但让同一个表中的另一列也有相同的值?

So you want to let one column use the auto_increment feature, but make another column in the same table also have the same value?

我想不出您需要此功能的原因.或许您可以解释一下您要实现的目标，我可以提出不同的解决方案?

I can't think of a reason you would need this feature. Perhaps you could explain what you're trying to accomplish, and I can suggest a different solution?

触发器对此不起作用.这是一个先有鸡还是先有蛋的问题:

A trigger won't work for this. It's a chicken-and-egg problem:

• 您不能在 AFTER 触发器中更改任何列的值.
• 但是当 BEFORE 触发器执行时，自动递增值尚未设置.

• You can't change any column's value in an AFTER trigger.
• But the auto-increment value isn't set yet when a BEFORE trigger executes.

使用 MySQL 5.7 GENERATED 列也不起作用:

It also won't work to use a MySQL 5.7 GENERATED column:

CREATE TABLE MyTable (
  id INT AUTO_INCREMENT PRIMARY KEY,
  why_would_you_want_this INT GENERATED ALWAYS AS (id)
);

ERROR 3109 (HY000): Generated column 'why_would_you_want_this' 
cannot refer to auto-increment column.

您不能在单个 SQL 语句中完成.您必须INSERT 行，然后立即执行UPDATE 以将第二列设置为相同的值.

You can't do it in a single SQL statement. You have to INSERT the row, and then immediately do an UPDATE to set your second column to the same value.

CREATE TABLE MyTable (
  id INT AUTO_INCREMENT PRIMARY KEY,
  why_would_you_want_this INT
);

INSERT INTO MyTable () VALUES ();

UPDATE MyTable SET why_would_you_want_this = LAST_INSERT_ID() 
WHERE id = LAST_INSERT_ID();

您也可以使用除 AUTO_INCREMENT 之外的其他机制(例如 Memcached 递增键)生成 ID 值.然后您可以在两列中插入新值:

You could alternatively generate the ID value using some other mechanism besides AUTO_INCREMENT (for example a Memcached incrementing key). Then you could insert the new value in both columns:

INSERT INTO MyTable (id, why_would_you_want_this) VALUES ($gen_id, $gen_id);

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