既存の MySQL テーブルのフィールドに PRIMARY KEY 制約を適用するにはどうすればよいでしょうか?

ALTER TABLE table_name MODIFY colum_name datatype PRIMARY KEY;
                 OR
ALTER TABLE table_name ADD PRIMARY KEY (colum_name); 

mysql> DESCRIBE Player;

+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| ID    |  int(11)    | YES  |     | NULL    |       |
| Name  | varchar(20) | YES  |     | NULL    |       |
+-------+-------------+------+-----+---------+-------+

2 rows in set (0.04 sec)
mysql> ALTER TABLE Player MODIFY ID INT PRIMARY KEY;
Query OK, 0 rows affected (0.22 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> DESCRIBE Player;

+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| ID    |  int(11)    | NO   | PRI | NULL    |       |
| Name  | varchar(20) | YES  |     | NULL    |       |
+-------+-------------+------+-----+---------+-------+

2 rows in set (0.04 sec) 

 Alter table Player ADD PRIMARY KEY(ID);