def METHOD_NAME(project_id=None) -> List[str]:
    """Return all project directories."""
    sql_dir = ConfigLoader.get("default", "sql_dir", fallback="sql")
    if project_id is None:
        return [
            os.path.join(sql_dir, project_dir) for project_dir in os.listdir(sql_dir)
        ]
    else:
        return [os.path.join(sql_dir, project_id)]