NULL不能和任何字段比较和运算

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