我的代码是这样的：

SELECT p.productid AS 'Redni broj',
    p.code AS 'Sifra robe',
    p.unitname AS 'Jedinica mere',
    di.quantity AS 'Kolicina',
    di.price 'Stara cena',
    di.itemvalue 'Stari iznos',
    di.price2 AS 'Nova cena',
    (di.price2 * di.quantity) AS 'Novi iznos',
    (di.price2 * di.quantity - di.itemvalue) AS 'Difference',
    SUM(di.price2 * di.quantity - di.itemvalue > 0) AS 'Positive difference',
    SUM(di.price2 * di.quantity - di.itemvalue < 0) AS 'Negative Difference'
FROM product AS p
LEFT JOIN productcategorypr AS pc ON p.productid = pc.productid
LEFT JOIN documentitem AS di ON p.productid = di.productid
GROUP BY p.productid

我需要过滤差异到2列，所有差异低于0，所有差异高于0，差异是新的价格-旧的价格。。。