Trying to list all articles where category field is empty
It seems to me your mistake is you are requesting the column field_tags from the table node, which is not a column from node, so doesn't exist.
Unless you have a very specific database structure, your "field_tags" should be contained in the table field_data_field_tags, so you may have to make an INNER JOIN between these tables in your query (maybe using field_data_field_tags.entity_id = n.id), and then check if the n.id exists in field_data_field_tags.entity_id, or (maybe better) use a LEFT JOIN between field_data_field_tags and node, so only the node with a n.id existing in field_data_field_tags will be returned.