Refine SQL table references for joins parameter

This issue highlights the possibility of enhancing the `convert(joins="")` parameter using conventions which may be more user friendly or recognizable by leveraging common syntax from other SQL dialects. At the moment, this statement makes references leveraging DuckDB SQL and references to intermediary files, which may not be as understandable as alternatives. Possible alternatives might include substitution variables (for ex. `&a_variable_name`).