Use defensive SQL clauses

Always use defensive SQL clauses like IF NOT EXISTS and IF EXISTS in migration scripts to make them idempotent and prevent failures when run multiple times or in different environments. This ensures migrations can be safely re-executed without causing errors due to existing schema elements.

Example:

-- Good: Uses IF NOT EXISTS to prevent errors
ALTER TABLE payment_intent ADD COLUMN IF NOT EXISTS enable_overcapture BOOLEAN;

-- Good: Uses IF EXISTS for safe removal
ALTER TABLE subscription DROP COLUMN IF EXISTS profile_id;

When performing complex operations like column renaming or constraint modifications, understand that some operations require separate statements and cannot be nested within a single ALTER TABLE command.