> For the complete documentation index, see [llms.txt](https://kelby.gitbook.io/rails-beginner-s-guide/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://kelby.gitbook.io/rails-beginner-s-guide/activerecord_migration/activerecord_migration_connectionadapters/connection-adapter_database_statements.md).

# Database Statements

常用的如：

```
execute
```

有时候，我们会在迁移文件里直接运行 SQL，使用上述方法，可以达到目的。

举例：

```ruby
class MakeJoinUnique < ActiveRecord::Migration
  def up
    execute "ALTER TABLE `pages_linked_pages` \n
             ADD UNIQUE `page_id_linked_page_id` (`page_id`,`linked_page_id`)"
  end

  def down
    execute "ALTER TABLE `pages_linked_pages` DROP INDEX `page_id_linked_page_id`"
  end
end
```

其它方法，不在此一一列举。