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Adding comments to the database

July 31 2008 : posted under documentation mysql sql

Comments in code are widely used and generally (but not universally) accepted as a good thing.

Databases can also accommodate comments - but these seem to be very rarely used.

I’m of the opinion that database comments are very useful.

It’s often hard to determine the exact meaning of a column - maybe it would be ideal to rename the column but you don’t want to risk breaking code.

I’ve also worked on a few projects where tables retain redundant columns just in case a query somewhere selects this column. Then every now and again a developer (like me) gets tripped up and thinks this column is meaningful.

For a really simple table

Create Table: CREATE TABLE `phonebook` (
  `number` int(11) NOT NULL COMMENT 'phone number',
  `name` varchar(255) default NULL,
  PRIMARY KEY  (`number`)
) ENGINE=MyISAM DEFAULT CHARSET=latin1
</code>

To add a comment to a table

alter table phonebook comment='an example table'

To add a comment to a column repeat the existing column definition and add the comment at the end

alter table phonebook
  change column name
    name varchar(255) default NULL comment 'full name'

The comments are then displayed when you show create table

It’s even easier to put the comments in at the start.


mysql&gt; create table phonebook (
  number int(11) primary key comment 'phone number',
  name varchar(255) comment 'full name'
  ) comment 'an example table';
Query OK, 0 rows affected (0.05 sec)

 show create table phonebook\G
*************************** 1. row ***************************
       Table: phonebook
Create Table: CREATE TABLE `phonebook` (
  `number` int(11) NOT NULL COMMENT 'phone number',
  `name` varchar(255) default NULL COMMENT 'full name',
  PRIMARY KEY  (`number`)
) ENGINE=MyISAM DEFAULT CHARSET=latin1 COMMENT='an example table'
1 row in set (0.00 sec)

The above is an example based on MySQL.