Syntax:

operand [NOT ]EXISTS (subquery)

The EXISTS operator returns TRUE if the subquery returns one or more rows and FALSE otherwise.

The NOT EXISTS operator returns TRUE if the subquery returns 0 rows and FALSE otherwise.

Finding all articles which have readers:

FROM Article
  WHERE EXISTS (FROM ReaderLog(id)
                WHERE ReaderLog.article_id = Article.id)

Finding all articles which don't have readers:

FROM Article
  WHERE NOT EXISTS (FROM ReaderLog(id)
                WHERE ReaderLog.article_id = Article.id)