MySQL触发器将字段更新为id的值

2021-11-20 00:00:00 insert mysql triggers

我想要一个触发器来对插入的记录执行以下操作:

I would like to have a trigger to perform following operation for inserted records:

 # pseudocode
 if new.group_id is null
    set new.group_id = new.id
 else
    # don't touch it
 end

更清楚:假设我有一个包含三列的表:id 主键、group_id int、value varchar.

More clearly: say I have one table with three columns: id primary key, group_id int, value varchar.

当我用 group_id 插入时:

INSERT INTO table(value, group_id) VALUES ('a', 10)

我想要:

id | group_id | value
---+----------+------
 1 |       10 | a

但是当我省略 group_id 时:

INSERT INTO table(value) VALUES ('b')

应该自动设置为这条记录的id:

it should be automatically set to the id of this record:

id | group_id | value
---+----------+------
 2 |        2 | b

可以用触发器吗?(我知道我可以在插入后更新记录，但有触发器会更好.)

Is it possible with a trigger? (I know I can update the record after inserting but having the trigger would be nicer.)

推荐答案

我不知道有什么方法可以在一个语句中做到这一点，即使使用触发器也是如此.

I don't know of any way to do this in one statement, even using a trigger.

@Lucky 建议的触发器解决方案在 MySQL 中如下所示:

The trigger solution that @Lucky suggested would look like this in MySQL:

CREATE TRIGGER MyTrigger BEFORE INSERT ON MyTable
FOR EACH ROW BEGIN
  SET NEW.group_id = COALESCE(NEW.group_id, NEW.id);
END

但是，有一个问题.在 BEFORE INSERT 阶段，自动生成的 id 值尚未生成.因此，如果 group_id 为 null，则默认为 NEW.id，始终为 0.

However, there's a problem. In the BEFORE INSERT phase, the auto-generated id value hasn't been generated yet. So if group_id is null, it defaults to NEW.id which is always 0.

但是如果你在AFTER INSERT阶段改变这个触发器触发，所以你可以访问NEW.id的生​​成值，你不能修改列值.

But if you change this trigger to fire during the AFTER INSERT phase, so you have access to the generated value of NEW.id, you can't modify column values.

MySQL 不支持列的 DEFAULT 表达式，因此您也不能在表定义中声明此行为.*更新:MySQL 8.0.13 支持 DEFAULT () 但表达式仍然不能依赖于自动递增值(这是 记录).

MySQL doesn't support expressions for the DEFAULT of a column, so you can't declare this behavior in the table definition either. *Update: MySQL 8.0.13 supports DEFAULT (<expression>) but the expression still can't depend on an auto-increment value (this is documented).

唯一的解决方案是执行INSERT，然后立即执行UPDATE 以更改group_id(如果未设置).>

The only solution is to do the INSERT, and then immediately do an UPDATE to change the group_id if it's not set.

INSERT INTO MyTable (group_id, value) VALUES (NULL, 'a');
UPDATE MyTable SET group_id = COALESCE(group_id, id) WHERE id = LAST_INSERT_ID();

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