MySQL Alter Table using existing field values

I am working with a database that has a table called date, which contains a separate field for day, month, year. Clearly this is not ideal when I am trying to run comparisons, etc. I am wondering is it possible for me to add a DateTime field to each row of this table and insert a concatenated string into the new field from the existing day, month, year fields.

I am quite sure its possible, I'm just wondering if anyone might be able to point me in the right direction on how to achieve this?

Below is the current date table: (i know)

CREATE TABLE IF NOT EXISTS `date` (
  `deposition_id` varchar(11) NOT NULL default '',
  `day` int(2) default NULL,
  `month` int(2) default NULL,
  `year` int(4) default NULL,
  PRIMARY KEY  (`deposition_id开发者_JS百科`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1;

first use alter table query:

alter table date add column datetimefield datetime NOT Null

then

use update query with self join on date and update datetimefield with concat on date,month, year column values.

Try this (untested) -

UPDATE date d SET d.datetime = (SELECT CONCAT('-','year','month','day') from date d1 where d1.id = d.id);

What is the problem, I don't understand? Alter the table, add new DATE column and then populate it with a string "yyyy-mm-dd" using CONCAT mysql function or whatever.