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.