Question

I am designing a message system.

In the table where I store the messages, the message_id column is auto increment.

I have another column message_reference_id.
I want its default value to be message_id.

How do I set this?

Answer 1

You can’t set the default value to the value in another column. Instead if your column is NOT NULL, you could change it to nullable and set the default value to NULL. Then when you select it, use this:

SELECT IFNULL(message_reference_id, message_id) AS message_reference_id
FROM your_table

You could even put this in a view.

Alternatively you can create a trigger which updates the value on insert.