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SQL - запрос, выражение после ORDER BY

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Всем привет. Написал такой Селект:

SELECT
	YEAR(action_date) AS `Year`,
    NULL AS `Month`,
    NULL AS `Day`,
    SUM(qty*price) AS `Total`
FROM actions
GROUP BY `Year` , `Month` , `Day`
UNION
SELECT 
	YEAR(action_date) AS `Year`,
    MONTH(action_date) AS `Month`,
    NULL AS `Day`,
    SUM(qty*price) AS `Total`
FROM actions
GROUP BY `Year` , `Month` , `Day`
UNION
SELECT
	YEAR(action_date) AS `Year`,
    MONTH(action_date) AS `Month`,
    DATE(action_date) AS `Day`,
    SUM(qty*price) AS `Total`
FROM actions
GROUP BY `Year` , `Month` , `Day`
ORDER BY `Year`, `Month`, `Day`

Результатом которого, выборка ниже:

'2015', NULL, NULL,        '181974.00000'
'2015', '7',  NULL,        '88319.00000'
'2015', '7', '2015-07-25', '35467.00000'
'2015', '7', '2015-07-26', '17149.00000'
'2015', '7', '2015-07-28', '8293.00000'
'2015', '7', '2015-07-29', '19760.00000'
'2015', '7', '2015-07-30', '7650.00000'

Подскажите, как можно заменить заменить NULL на пустые значения? И как можно выполнять модификации над выборкой, после ORDER BY? Заранее спасибо.