UPDATE dbo.PayPalPaymentInfo
SET GrossAmount=TotalPrice+TaxAmount
WHERE GrossAmount IS NULL



--如果TaxAmount为NUll时,TotalPrice有值,会导致GrossAmount也为null
--即NULL+任何数字都会是NULL
--NULL不能和任何字段比较和运算
UPDATE dbo.PayPalPaymentInfo
SET GrossAmount=TotalPrice+ISNULL(TaxAmount,0)
WHERE GrossAmount IS NULL