add column to mysql table if it does not exist

Question

My research and experiments haven't yielded an answer yet, so I am hoping for some help.

I am modifying the install file of an application which in previous versions did not have a column which I want to add now. I do not want to add the column manually, but in the installation file and only if the new column does not already exist in the table.

The table is created as follows:

CREATE TABLE IF NOT EXISTS `#__comm_subscribers` (
      `subscriber_id` int(11) NOT NULL auto_increment,
      `user_id` int(11) NOT NULL default '0',
      `subscriber_name` varchar(64) NOT NULL default '',
      `subscriber_surname` varchar(64) NOT NULL default '',
      `subscriber_email` varchar(64) NOT NULL default '',
      `confirmed` tinyint(1) NOT NULL default '0',
      `subscribe_date` datetime NOT NULL default '0000-00-00 00:00:00',
      PRIMARY KEY  (`subscriber_id`),
      UNIQUE KEY `subscriber_email` (`subscriber_email`)
    ) ENGINE=MyISAM CHARACTER SET 'utf8' COLLATE 'utf8_general_ci' COMMENT='Subscribers for Comm are stored here.';

If I add the following, below the create table statement, then I am not sure what happens if the column already exists (and perhaps is populated):

ALTER TABLE `#__comm_subscribers` ADD `subscriber_surname`;
ALTER TABLE `#__comm_subscribers` MODIFY `subscriber_surname` varchar(64) NOT NULL default '';

So, I tried the following which I found somewhere. This does not seem to work but I am not entirely sure I used it properly.

/*delimiter '//'
CREATE PROCEDURE addcol() BEGIN
IF NOT EXISTS(
SELECT * FROM information_schema.COLUMNS
WHERE COLUMN_NAME='subscriber_surname' AND TABLE_NAME='#__comm_subscribers'
)
THEN
 ALTER TABLE `#__comm_subscribers`
 ADD COLUMN `subscriber_surname` varchar(64) NOT NULL default '';
END IF;
END;
//
delimiter ';'
CALL addcol();
DROP PROCEDURE addcol;*/

Does anyone have a good way to do this?

Answer 1

Note that INFORMATION_SCHEMA isn't supported in MySQL prior to 5.0. Nor are stored procedures supported prior to 5.0, so if you need to support MySQL 4.1, this solution isn't good.

One solution used by frameworks that use database migrations is to record in your database a revision number for the schema. Just a table with a single column and single row, with an integer indicating which revision is current in effect. When you update the schema, increment the number.

Another solution would be to just try the ALTER TABLE ADD COLUMN command. It should throw an error if the column already exists.

ERROR 1060 (42S21): Duplicate column name 'newcolumnname'

Catch the error and disregard it in your upgrade script.

Answer 2

Just tried the stored procedure script. Seems the problem is the ' marks around the delimiters. The MySQL Docs show that delimiter characters do not need the single quotes.

So you want:

delimiter //

Instead of:

delimiter '//'

Works for me :)

Answer 3

Here is a working solution (just tried out with MySQL 5.0 on Solaris):

DELIMITER $$

DROP PROCEDURE IF EXISTS upgrade_database_1_0_to_2_0 $$
CREATE PROCEDURE upgrade_database_1_0_to_2_0()
BEGIN

-- rename a table safely
IF NOT EXISTS( (SELECT * FROM information_schema.COLUMNS WHERE TABLE_SCHEMA=DATABASE()
        AND TABLE_NAME='my_old_table_name') ) THEN
    RENAME TABLE 
        my_old_table_name TO my_new_table_name,
END IF;

-- add a column safely
IF NOT EXISTS( (SELECT * FROM information_schema.COLUMNS WHERE TABLE_SCHEMA=DATABASE()
        AND COLUMN_NAME='my_additional_column' AND TABLE_NAME='my_table_name') ) THEN
    ALTER TABLE my_table_name ADD my_additional_column varchar(2048) NOT NULL DEFAULT '';
END IF;

END $$

CALL upgrade_database_1_0_to_2_0() $$

DELIMITER ;

On a first glance it probably looks more complicated than it should, but we have to deal with following problems here:

• IF statements only work in stored procedures, not when run directly, e.g. in mysql client
• more elegant and concise SHOW COLUMNS does not work in stored procedure so have to use INFORMATIONAL_SCHEMA
• the syntax for delimiting statements is strange in MySQL, so you have to redefine the delimiter to be able to create stored procedures. Do not forget to switch the delimiter back!
• INFORMATIONAL_SCHEMA is global for all databases, do not forget to filter on TABLE_SCHEMA=DATABASE(). DATABASE() returns the name of the currently selected database.