mysql> DROP INDEX b ON t21126;
Query OK, 4 rows affected (0.02 sec)
Records: 4  Duplicates: 0  Warnings: 0

mysql> SELECT * FROM t21126 WHERE b LIKE 'あいうえおかきくけこ%';
+---+------------------------------------------+------+------+------+------+------+------+------+------+------+------+------+------+
| a | b                                        | c    | d    | e    | f    | g    | h    | i    | j    | k    | l    | m    | n    |
+---+------------------------------------------+------+------+------+------+------+------+------+------+------+------+------+------+
| 2 | あいうえおかきくけこ                     | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | 
| 3 | あいうえおかきくけこさしすせそ           | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | 
| 4 | あいうえおかきくけこさしすせそたちつてと | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | 
+---+------------------------------------------+------+------+------+------+------+------+------+------+------+------+------+------+
3 rows in set (0.00 sec)

mysql> ALTER TABLE t21126 ADD UNIQUE (b);
Query OK, 4 rows affected (0.03 sec)
Records: 4  Duplicates: 0  Warnings: 0

mysql> SELECT * FROM t21126 WHERE b LIKE 'あいうえおかきくけこ%';
+---+----------------------+------+------+------+------+------+------+------+------+------+------+------+------+
| a | b                    | c    | d    | e    | f    | g    | h    | i    | j    | k    | l    | m    | n    |
+---+----------------------+------+------+------+------+------+------+------+------+------+------+------+------+
| 2 | あいうえおかきくけこ | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | 
+---+----------------------+------+------+------+------+------+------+------+------+------+------+------+------+
1 row in set (0.00 sec)