Articles
id | text

Keywords
id | word

Articles_keywords
id_article | id_keyword

SELECT * FROM Articles AS a WHERE a.id IN
    (SELECT id_article FROM Articles_Keywords AS ak WHERE ak.id_keyword IN
        (SELECT id FROM keywords AS k WHERE k.word = 'xyz'));

SELECT * FROM Articles as a
    JOIN Articles_Keywords as ak
        ON a.id = ak.id_article
    JOIN Keywords as k
        ON k.id = ak.id_keyword
WHERE k.word = 'xyz';