mysql> select CAST(concat(myyear,'-',mymonth,'-',myday) as DATE) from mytable; 
+----------------------------------------------------+
| CAST(concat(myyear,'-',mymonth,'-',myday) as DATE) |
+----------------------------------------------------+
| 2000-05-14                                         | 
| NULL                                               | 
| NULL                                               | 
| NULL                                               | 
+----------------------------------------------------+
4 rows in set, 3 warnings (0.00 sec)

mysql> show warnings;
+---------+------+--------------------------------------------------+
| Level   | Code | Message                                          |
+---------+------+--------------------------------------------------+
| Warning | 1292 | Truncated incorrect datetime value: '2004-12-34' | 
| Warning | 1292 | Truncated incorrect datetime value: '1997-15-12' | 
| Warning | 1292 | Truncated incorrect datetime value: '2005-25-1'  | 
+---------+------+--------------------------------------------------+
3 rows in set (0.00 sec)