Combinación de MySQL Dos campos de dos tablas en un campo en el conjunto de resultados

SELECT  *
FROM    (
        SELECT  itemid, key, iv.value, s.value AS svalue
        FROM    integervalues iv
        JOIN    strings s
        ON      s.id = iv.value
        WHERE   iv.itemid = 1
        UNION ALL
        SELECT  itemid, key, NULL, value
        FROM    stringvalues sv
        WHERE   sv.itemid = 1
        ) vals
JOIN    items i
ON      i.id = vals.itemid