MySQL默认值为其他字段的值

Can I, and, if I can, how can I set the default value of a field in a MySQL table to the value of another field?

我可以，如果可以的话，我如何将MySQL表中字段的默认值设置为另一个字段的值？

Thing is: I have data, and each data object has its ID in the table. But, I would like the possibility to rearrange the data, changing their sorting index, without altering their ID. Thus, the field sort_num should by default be set to the value given to the auto-incremented indexed field ID.

事情是：我有数据，每个数据对象在表中都有自己的ID。但是，我希望有可能重新排列数据，改变他们的排序索引，而不改变他们的ID。因此，默认情况下，字段sort_num应设置为赋予自动递增的索引字段ID的值。

Thanks in advance!

提前致谢！

2 个解决方案

#1

I see two possible solutions for this:

我看到两个可能的解决方案：

1. Possibility:

1.可能性：

You use a function to simply ignore sort_num if it is not set:

如果未设置，则使用函数简单地忽略sort_num：

`SELECT * FROM mytable ORDER BY coalesce(sort_num, id)`

coalesce() returns the first non-null value, therefore you would insert values for sort_num if you really need to reorder items.

coalesce（）返回第一个非null值，因此如果确实需要重新排序项，则应插入sort_num的值。

2. Possibility:

2.可能性：

You write a trigger, which automatically sets the value if it is not set in the insert statement:

您编写了一个触发器，如果未在insert语句中设置该值，则会自动设置该值：

DELIMITER //

CREATE TRIGGER sort_num_trigger
BEFORE INSERT ON mytable
FOR EACH ROW BEGIN
    DECLARE auto_inc INT;
    IF (NEW.sort_num  is null) THEN
         -- determine next auto_increment value
        SELECT AUTO_INCREMENT INTO auto_inc FROM information_schema.TABLES
        WHERE TABLE_SCHEMA=DATABASE() AND TABLE_NAME = 'mytable';
        -- and set the sort value to the same as the PK
        SET NEW.sort_num = auto_inc;
    END IF;
END
//

(inspired by this comment)

（受此评论的启发）

However, this might run into parallelization issues (multiple queries inserting at the same time)

但是，这可能会遇到并行化问题（同时插入多个查询）

#2

Its a bad idea to have an auto-increment column rearranged, hence better idea would be

将一个自动增量列重新排列是一个坏主意，因此更好的想法

Add a column sort_num to the table

向表中添加列sort_num

ALTER TABLE data ADD sort_num column-definition; (column definition same as ID)

UPDATE data SET sort_num = ID

Now play with sort_num column as it has no effect on column ID

现在使用sort_num列，因为它对列ID没有影响

#1