Bon, J'ai trouvé quelque chose qui marche:
SELECT id, pn, pv1, NULL AS pv2, NULL AS pv3, NULL AS pv4
FROM object_property_int
WHERE object_property_int.id
IN ( '1' )
UNION SELECT id, pn, NULL AS pv1, pv2, NULL AS pv3, NULL AS pv4
FROM object_property_text
WHERE object_property_text.id
IN ( '1' )
UNION SELECT id, pn, NULL AS pv1, NULL AS pv2, pv3, NULL AS pv4
FROM object_property_datetime
WHERE object_property_datetime.id
IN ( '1' )
UNION SELECT id, pn, NULL AS pv1, NULL AS pv2, NULL AS pv3, pv4
FROM object_property_varchar100
WHERE object_property_varchar100.id
IN ( '1' )
LIMIT 0 , 30