I second this - migrations in Rails are fantastic. Even if you aren't using Rails you can get the styles and source code here https://edgeguides.rubyonrails.org/active_record_migrations....

Large open source projects such as GitLab source code are also treasures to discover migration rules.

If you have a millions of users with high traffic tables you may want to check some migration guides for downtime, data migrations and batch processing here https://docs.gitlab.com/ee/development/migration_style_guide...

If you want to have some concurrent ways of DDL operations you may benefit from https://gitlab.com/gitlab-org/gitlab/blob/master/lib%2Fgitla...