where score IS NULL
where score IS NOT NULL
空值返回0以便计算
SELECT ProductName,UnitPrice*(UnitsInStock+ISNULL(UnitsOnOrder,0)) FROM Products
SELECT ProductName,UnitPrice*(UnitsInStock+NVL(UnitsOnOrder,0)) FROM Products
SELECT ProductName,UnitPrice*(UnitsInStock+IFNULL(UnitsOnOrder,0)) FROM Products SELECT ProductName,UnitPrice*(UnitsInStock+COALESCE(UnitsOnOrder,0)) FROM Products
原文：https://www.cnblogs.com/soberkkk/p/12574470.html