You could also use a simple correlated subquery if all you wanted was the concat of the applicable keywords:
```
select docs.id,
docs.title AS Title,
authors.author AS Author,
docs.source) AS Source,
(
select group_concat(keyword)
from keywords
where docid == D.docid
and authorid == D.Authorid
) as Keywords
from docs, authors
where docs.authorid == authors.authorid
and docs.docid == author.docid
order by docs.id desc
limit 5
;
```