Why there can be only one TIMESTAMP column with CURRENT_TIMESTAMP in DEFAULT clause?

Why there can be only one TIMESTAMP column with CURRENT_TIMESTAMP in DEFAULT or ON UPDATE clause?

CREATE TABLE `foo` (
  `ProductID` INT(10) UNSIGNED NOT NULL,
  `AddedDate` TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP,
  `UpdatedDate` TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP
) ENGINE=INNODB;

The error that results:

Error Code : 1293

Incorrect table definition; there can be only one TIMESTAMP column with CURRENT_TIMESTAMP in DEFAULT or ON UPDATE clause

Solution 1:

This limitation, which was only due to historical, code legacy reasons, has been lifted in recent versions of MySQL:

Changes in MySQL 5.6.5 (2012-04-10, Milestone 8)

Previously, at most one TIMESTAMP column per table could be automatically initialized or updated to the current date and time. This restriction has been lifted. Any TIMESTAMP column definition can have any combination of DEFAULT CURRENT_TIMESTAMP and ON UPDATE CURRENT_TIMESTAMP clauses. In addition, these clauses now can be used with DATETIME column definitions. For more information, see Automatic Initialization and Updating for TIMESTAMP and DATETIME.

http://dev.mysql.com/doc/relnotes/mysql/5.6/en/news-5-6-5.html

Solution 2:

I also wondered that long time ago. I searched a bit in my history and I think that this post: http://lists.mysql.com/internals/34919 represents the semi-official position of MySQL (before Oracle's intervention ;))

In short:

this limitation stems only from the way in which this feature is currently implemented in the server and there are no other reasons for its existence.

So their explanation is "because it is implemented like this". Doesn't sound very scientific. I guess it all comes from some old code. This is suggested in the thread above: "carry-over from when only the first timestamp field was auto-set/update".

Cheers!