The answer to this might just be that it can't be done but if I have a table with a datetime type field, that I want to automatically update whenever the record is updated. Now if the type was timestamp this could be done as follows:
ALTER TABLE `mytable` CHANGE `modified_date`, 'modified_date' TIMESTAMP ON UPDATE CURRENT_TIMESTAMP NOT NULL
However I can't get this to work with a DATETIME field. I have tried variations such as NOW in place of CURRENT_TIMESTAMP with no luck.
Presumably this can be done with triggers, but I'd like to be able to do it via the basic attributes.