How can we apply the PRIMARY KEY constraint to the field of an existing MySQL table?

We can apply the PRIMARY KEY constraint to a column of an existing MySQL table with the help of ALTER TABLE statement. 

Syntax

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

Suppose we have the following table named ‘Player’ and we want to add the PRIMARY KEY constraint to the column ‘ID’ then it can be done with the help of ALTER TABLE command as follows −

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) 

From the above result set, it can be observed that MySQL added a PRIMARY KEY constraint to the filed ‘ID’. We can add a PRIMARY KEY constraint with the following query as well −

 Alter table Player ADD PRIMARY KEY(ID);