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In MySQL, IF statements cannot exist outside of stored procedures. Therefore, to create an idempotent migration for MySQL it's necessary to wrap the migration in a stored procedure and execute that stored procedure against the database to perform the migration.
DROP PROCEDURE IF EXISTS add_email_address_column_to_customers_table $$
-- Create the stored procedure to perform the migration
CREATE PROCEDURE add_email_address_column_to_customers_table()
-- Add the email_address column to the customers table, if it doesn't already exist
IF NOT EXISTS ((SELECT * FROM information_schema.columns WHERE table_schema=DATABASE() AND table_name='customers' AND column_name='email_address')) THEN
ALTER TABLE customers ADD email_address VARCHAR(256);
END $$
-- Execute the stored procedure
CALL add_email_address_column_to_customers_table() $$
-- Don't forget to drop the stored procedure when you're done!
DROP PROCEDURE IF EXISTS add_email_address_column_to_customers_table $$
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