How Can I Conditionally Add a Column to a MySQL Table Using `ALTER TABLE`?

MySQL: Conditional Column Addition Using ALTER TABLE IF NOT EXISTS

In MySQL, modifying a table structure often requires adding or removing columns. However, such operations can fail if the specified column already exists or does not exist. To address this scenario, MySQL provides a conditional way to add a column only if it does not exist using the ALTER TABLE IF NOT EXISTS syntax.

Problem Statement

Consider the following task: altering a table named settings to add a column called multi_user as a TINYINT(1) with a default value of 1, but only if the column does not already exist. Many attempts using different approaches, such as the ADD COLUMN IF NOT EXISTS syntax or a stored procedure with a conditional IF statement, have failed.

Solution

To conditionally add a column using ALTER TABLE IF NOT EXISTS, follow these steps:

1. Create a stored procedure that encapsulates the logic.
2. Within the stored procedure, use the following conditional query to check if the column exists:

IF NOT EXISTS( SELECT NULL
                FROM INFORMATION_SCHEMA.COLUMNS
               WHERE table_name = 'settings'
                 AND table_schema = 'database_name'
                 AND column_name = 'multi_user')  THEN

3. If the column does not exist, add the column using the ALTER TABLE statement:

  ALTER TABLE `settings` ADD `multi_user` int(1) NOT NULL default '0';

Example

Here is an example of the stored procedure you can create:

DELIMITER $$
CREATE PROCEDURE Alter_Table()
BEGIN
  IF NOT EXISTS( SELECT NULL
                  FROM INFORMATION_SCHEMA.COLUMNS
                 WHERE table_name = 'settings'
                   AND table_schema = 'database_name'
                   AND column_name = 'multi_user')  THEN
    
    ALTER TABLE `settings` ADD `multi_user` int(1) NOT NULL default '0';
  END IF;
END $$
DELIMITER ;

This procedure will check if the multi_user column exists in the settings table. If it does not exist, the column will be added. Otherwise, the operation will be skipped.

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