不确定如何应用isNull（x，0）

Question

在Microsoft SQL Server 2008中，我想执行以下操作：

UPDATE dbo.Test SET
Earned=(
SELECT Points
FROM dbo.Answer
WHERE AnswerID = Guess_AnswerID
AND Correct=1
)

问题是当correct = 0时，select为null，而Earned需要为0而不是null。

Answer 1

在任何2个表达式周围添加IsNull。在这种情况下，您也可以使用Coalesce。

UPDATE dbo.Test SET
Earned=IsNull((
    SELECT Points
    FROM dbo.Answer
    WHERE AnswerID = Guess_AnswerID
    AND Correct=1
), 0)

您可能还希望将每个匹配答案的所有要点相加。如果是这种情况，则需要将其设为Sum（Points）而不仅仅是Points。

UPDATE dbo.Test SET
Earned=IsNull((
    SELECT Sum(Points)
    FROM dbo.Answer
    WHERE AnswerID = Guess_AnswerID
    AND Correct=1
), 0)

Answer 2

如果正确= 0

，这将有效

 UPDATE dbo.Test SET
Earned=(
SELECT isnull(Points,0)
FROM dbo.Answer
WHERE AnswerID = Guess_AnswerID
)

Answer 3

update t
set Earned=case a.Correct when 1 then a.Points else 0 end
from dbo.Test t
inner join dbo.Answer a on a.AnswerID = Guess_AnswerID