To powinno działać dla Ciebie:
UPDATE table2 T
SET T.accessories = IFNULL((SELECT foo FROM
(SELECT t1.ProductID,GROUP_CONCAT(t2.GRP) as foo
FROM table1 t1
LEFT JOIN table2 t2 ON t1.Accessory = t2.ProductID
GROUP BY t1.ProductID
) AS bar WHERE ProductID = T.ProductID
),'')
Oto zaktualizowany SQL Fiddle
