How to alter the data type of a MySQL table’s column?

You can use modify command for this. Let us first us create a table.

mysql> create table DemoTable
(
   StudentId varchar(200) not null,
   StudentName varchar(20),
   StudentAge int,
   StudentAddress varchar(20),
   StudentCountryName varchar(20)
);
Query OK, 0 rows affected (0.73 sec)

Now check the description of table.

mysql> desc DemoTable;

This will produce the following output −

+--------------------+--------------+------+-----+---------+-------+
| Field              | Type         | Null | Key | Default | Extra |
+--------------------+--------------+------+-----+---------+-------+
| StudentId          | varchar(200) | NO   |     | NULL    |       |
| StudentName        | varchar(20)  | YES  |     | NULL    |       |
| StudentAge         | int(11)      | YES  |     | NULL    |       |
| StudentAddress     | varchar(20)  | YES  |     | NULL    |       |
| StudentCountryName | varchar(20)  | YES  |     | NULL    |       |
+--------------------+--------------+------+-----+---------+-------+
5 rows in set (0.00 sec)

Here is the query to alter the data type from varchar to BIGINT −

mysql> alter table DemoTable modify StudentId BIGINT(10) UNSIGNED NOT NULL DEFAULT 0;
Query OK, 0 rows affected (1.51 sec)
Records: 0 Duplicates : 0 Warnings : 0

Above, we have changed the column ‘StudentId’ datatype from varchar(200) to BIGINT(10). Let us check the description of table once again.

mysql> desc DemoTable;

This will produce the following output −

+--------------------+---------------------+------+-----+---------+-------+
| Field              | Type                | Null | Key | Default | Extra |
+--------------------+---------------------+------+-----+---------+-------+
| StudentId          | bigint(10) unsigned | NO   |     | 0       |       |
| StudentName        | varchar(20)         | YES  |     | NULL    |       |
| StudentAge         | int(11)             | YES  |     | NULL    |       |
| StudentAddress     | varchar(20)         | YES  |     | NULL    |       | 
| StudentCountryName | varchar(20)         | YES  |     | NULL    |       |
+--------------------+---------------------+------+-----+---------+-------+
5 rows in set (0.00 sec)