Set NOW() as Default Value for datetime datatype?

As of MySQL 5.6.5, you can use the DATETIME type with a dynamic default value:

CREATE TABLE foo (    creation_time      DATETIME DEFAULT   CURRENT_TIMESTAMP,    modification_time  DATETIME ON UPDATE CURRENT_TIMESTAMP)

Or even combine both rules:

modification_time DATETIME DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP

Reference:
http://dev.mysql.com/doc/refman/5.7/en/timestamp-initialization.html
http://optimize-this.blogspot.com/2012/04/datetime-default-now-finally-available.html

Prior to 5.6.5, you need to use the TIMESTAMP data type, which automatically updates whenever the record is modified. Unfortunately, however, only one auto-updated TIMESTAMP field can exist per table.

CREATE TABLE mytable (  mydate TIMESTAMP)

See: http://dev.mysql.com/doc/refman/5.1/en/create-table.html

If you want to prevent MySQL from updating the timestamp value on UPDATE (so that it only triggers on INSERT) you can change the definition to:

CREATE TABLE mytable (  mydate TIMESTAMP DEFAULT CURRENT_TIMESTAMP)

I use a trigger as a workaround to set a datetime field to NOW() for new inserts:

CREATE TRIGGER `triggername` BEFORE INSERT ON  `tablename` FOR EACH ROW SET NEW.datetimefield = NOW()

it should work for updates too

Answers by Johan & Leonardo involve converting to a timestamp field. Although this is probably ok for the use case presented in the question (storing RegisterDate and LastVisitDate), it is not a universal solution. See datetime vs timestamp question.

My solution

ALTER TABLE `table_name` MODIFY COLUMN `column_name` TIMESTAMP NOTNULL DEFAULT CURRENT_TIMESTAMP;