Workaround for CURRENT_TIMESTAMP as default for a DATETIME value

Currently you cannot specify the current time as a default value for a DATETIME column. It works for TIMESTAMP but not for DATETIME,and even with TIMESTAMP this only work for the one timestamp column in the table, if you have more than 1 TIMESTAMP column you have to specify a default for all but the first.

This is fixed in MySQL 5.6 and MariaDB 10 which allows you to use a construct like this:
mysql> CREATE TABLE td1(
  c1 INT NOT NULL PRIMARY KEY AUTO_INCREMENT,
  c2 DATETIME DEFAULT NULL());

So what can you do if you are on MariaDB? Well, you can use a trigger to achieve a similar result:
mysql> CREATE TABLE td1(
  c1 INT NOT NULL PRIMARY KEY AUTO_INCREMENT,
  c2 DATETIME DEFAULT NULL());
delimiter //
mysql> CREATE TRIGGER td1_ins BEFORE INSERT ON td1 FOR EACH ROW
BEGIN
  IF new.c2 IS NULL THEN
    SET new.c2 = now();
  END IF;
END// 
delimiter ;

And then this works:
mysql> INSERT INTO td1 VALUES(NULL, '1960-04-10 16:45');
mysql> INSERT INTO td1 VALUES(NULL, NULL);
mysql> INSERT INTO td1(c1) VALUES(NULL);
mysql> SELECT * FROM td1;
+----+---------------------+
| c1 | c2                  |
+----+---------------------+
|  1 | 1960-01-04 16:45:00 |
|  2 | 2013-11-23 15:57:51 |
|  3 | 2013-11-23 16:00:55 |
+----+---------------------+
4 rows in set (0.00 sec)


Is this perfect? Nope, but it works while we wait for MariaDB 10.

/Karlsson