How To Add a New Column to an Existing Table? - MySQL FAQs - Understanding SQL CREATE, ALTER and DROP Statements

If you have an existing table with existing data rows, and want to add a new column to that table, you can use the "ALTER TABLE ... ADD COLUMN" statement. The tutorial script below shows you a good example:

mysql> ALTER TABLE tip ADD COLUMN author VARCHAR(40);
Query OK, 1 row affected (0.18 sec)
Records: 1  Duplicates: 0  Warnings: 0

mysql> SHOW COLUMNS FROM tip;
+-------------+--------------+------+-----+---------+-------
| Field       | Type         | Null | Key | Default | Extra 
+-------------+--------------+------+-----+---------+-------
| id          | int(11)      | NO   | PRI |         |       
| subject     | varchar(80)  | NO   |     |         |       
| description | varchar(256) | NO   |     |         |       
| create_date | date         | YES  |     | NULL    |       
| author      | varchar(40)  | YES  |     | NULL    |       
+-------------+--------------+------+-----+---------+-------
5 rows in set (0.01 sec)

This SQL script added a new column called "author" to the "tip" table. NULL values were added to this column on all existing data rows.