MYSQL объединение UNION по ID

Как сделать подобную выборку, получив уникальный ID?

SELECT offer_id, value AS art, NULL AS name
FROM shop_item_param_text AS art
WHERE art.param_id = 1
UNION
SELECT offer_id, NULL AS art, value AS name
FROM shop_item_param_text AS name
WHERE name.param_id = 2