Próbáltam én a tagadást, csak nem az AND NOT-al...ezért nem működött
Akkor ez így jó?:
SELECT p.products_model, p.products_price, pd.products_name, pd.products_description, pdo.products_description AS old_description, pd.language_id, p.products_last_modified
FROM products p, products_description pd, products_description_old AS pdo
WHERE pdo.products_id = pd.products_id
AND p.products_id = pd.products_id
AND pd.language_id = pdo.language_id
AND NOT pd.products_description <=> pdo.products_description
Köszi 
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