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Add a Column to a table if not exists

MySQL allows you to create a table if it does not exist, but does not provide a native way of a adding a column (i.e. a field) to an existing table with a test of whether the column already exists - so as to avoid an error if the column already exists. The ability to add a column if it does not exist can be useful for upgrade scripts and the like.

The following script creates a stored procedure that allows a column to be added to a table but only if it does not already exist:

-- Copyright (c) 2009
-- Script is free to use provided this copyright header is included.
drop procedure if exists AddColumnUnlessExists;
delimiter '//'

create procedure AddColumnUnlessExists(
	IN dbName tinytext,
	IN tableName tinytext,
	IN fieldName tinytext,
	IN fieldDef text)
		SELECT * FROM information_schema.COLUMNS
		WHERE column_name=fieldName
		and table_name=tableName
		and table_schema=dbName
		set @ddl=CONCAT('ALTER TABLE ',dbName,'.',tableName,
			' ADD COLUMN ',fieldName,' ',fieldDef);
		prepare stmt from @ddl;
		execute stmt;

delimiter ';'

This stored procedure provides the functionality for "add column if not exists". To use the script call it with the name of the database, name of the table, name of the field and the field definition to use if the field is to be created. For example:

call AddColumnUnlessExists(Database(), 'accounts', 'dob', 'varchar(32) null');

which will add the field "dob" to the table "accounts" in the current database, unless it already exists, or

call AddColumnUnlessExists('GIS', 'boundaries', 'fillColour', 'int unsigned not null default 1');

which will add the field "fillColour" to the table "boundaries" in the database "GIS" if it does not already exist.

If you want to drop the stored procedure after use then use:

drop procedure AddColumnUnlessExists;

These notes have been tested against MySQL version 5.1.

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