Although it may seem so, they do NOT do exactly same thing.
While GROUP BY
groups results only by those columns that are explicitly listed after the clause, DISTINCT
groups results by ALL columns that are present in SELECT
statement (and not only by the one it precedes).
So following queries
SELECT DISTINCT article_id FROM article_authors
SELECT article_id FROM article_authors GROUP BY article_id
return same results,
but
SELECT DISTINCT article_id, author_id FROM article_authors
SELECT article_id FROM article_authors GROUP BY article_id
return different results,
though
SELECT DISTINCT article_id, author_id FROM article_authors
SELECT article_id FROM article_authors GROUP BY article_id, author_id
again return same results.
As a sidenote, with GROUP BY
you haveĀ HAVING
clause by your hand as well.