Question

I’m creating a messaging system (using PHP) and want to assign an ID number to each message (aside from each actual message having an unique ID number)…however, if someone replies to a message then i want to be able to give that message the same ID as the message being replied to…then of course I can disply them by time and show them in order.

So, if i give the field an auto_increment type is that able to be overwritten?

Meaning…each new message has auto value e.g. 1, 2, 3 etc but someone replies to number 2 so it’s ID needs to also 2

Or is there a better way to do this?

Answer 1

Absolutely nothing prevents you from assigning any arbitrary value to an AUTO_INCREMENT column. If necessary, the table counter will adjust accordingly.

However, you cannot set as AUTO_INCREMENT a column that’s not unique.

Honestly, I can’t understand your design. A typical messaging system would look like this:

message_id in_reply_to
========== ===========
         1        NULL
         2        NULL
         3           1
         4        NULL
         5           1
         6           3
         7        NULL

Duplicating IDs kind of beats the purpose of using IDs.

Update #1: OMG, it seems that it can actually be done under certain circumstances:

For MyISAM tables you can specify AUTO_INCREMENT on a secondary column
in a multiple-column index. In this case, the generated value for the
AUTO_INCREMENT column is calculated as MAX(auto_increment_column) + 1
WHERE prefix=given-prefix. This is useful when you want to put data
into ordered groups.

http://dev.mysql.com/doc/refman/5.5/en/example-auto-increment.html

Update #2: For the records, I’ve just tested it and you can use duplicate auto-incremented IDs in InnoDB tables as well:

CREATE TABLE foo (
    foo_id INT(10) NOT NULL DEFAULT '0',
    bar_id INT(10) NOT NULL AUTO_INCREMENT,
    PRIMARY KEY (foo_id),
    INDEX bar_id (bar_id)
)
ENGINE=InnoDB