「DISTINCT」句で重複するデータを一つにまとめて表示

SQL 
mysql> SELECT ndate FROM t_hanbai;
+------------+
| ndate      |
+------------+
| 2005-07-11 |
| 2005-07-12 |
| 2005-07-12 |
| 2005-07-21 |
| 2005-07-22 |
+------------+
5 rows in set (0.00 sec)

mysql> SELECT DISTINCT ndate FROM t_hanbai;
+------------+
| ndate      |
+------------+
| 2005-07-11 |
| 2005-07-12 |
| 2005-07-21 |
| 2005-07-22 |
+------------+
4 rows in set (0.00 sec)