[mysql] Alter MySQL table to add comments on columns

You can use MODIFY COLUMN to do this. Just do...

ALTER TABLE YourTable
MODIFY COLUMN your_column
your_previous_column_definition COMMENT "Your new comment"

substituting:

• YourTable with the name of your table
• your_column with the name of your comment
• your_previous_column_definition with the column's column_definition, which I recommend getting via a SHOW CREATE TABLE YourTable command and copying verbatim to avoid any traps.*
• Your new comment with the column comment you want.

For example...

mysql> CREATE TABLE `Example` (
    ->   `id` int(10) unsigned NOT NULL AUTO_INCREMENT,
    ->   `some_col` varchar(255) DEFAULT NULL,
    ->   PRIMARY KEY (`id`)
    -> );
Query OK, 0 rows affected (0.18 sec)

mysql> ALTER TABLE Example
    -> MODIFY COLUMN `id`
    -> int(10) unsigned NOT NULL AUTO_INCREMENT COMMENT 'Look, I''m a comment!';
Query OK, 0 rows affected (0.07 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> SHOW CREATE TABLE Example;
+---------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table   | Create Table                                                                                                                                                                                                  |
+---------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Example | CREATE TABLE `Example` (
  `id` int(10) unsigned NOT NULL AUTO_INCREMENT COMMENT 'Look, I''m a comment!',
  `some_col` varchar(255) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1 |
+---------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

* Whenever you use MODIFY or CHANGE clauses in an ALTER TABLE statement, I suggest you copy the column definition from the output of a SHOW CREATE TABLE statement. This protects you from accidentally losing an important part of your column definition by not realising that you need to include it in your MODIFY or CHANGE clause. For example, if you MODIFY an AUTO_INCREMENT column, you need to explicitly specify the AUTO_INCREMENT modifier again in the MODIFY clause, or the column will cease to be an AUTO_INCREMENT column. Similarly, if the column is defined as NOT NULL or has a DEFAULT value, these details need to be included when doing a MODIFY or CHANGE on the column or they will be lost.