我有以下声明:
SELECT COUNT(AccedentId) AS NumberOfAccedentInYear
FROM Accident
GROUP BY DriverId, YEAR(AccedentDate)
HAVING (DriverId =@DriverId)<3
当有行时, 我能得到一个有效的计数, 但是当没有行时, 结果是空的 。 如果没有匹配的行, 我希望结果为 0 。
也许你想要这样(我保留明显的语法和拼写错误,
SELECT NumberOfAccedentInYear = ISNULL
(
(SELECT COUNT(AccedentId)
FROM Accident
GROUP BY DriverId, YEAR(AccedentDate)
HAVING (DriverId =@DriverId)<3))
, 0
);
对于任何对COALESCE诉ISNULLL的讨论感到好奇的人, 以及为什么我改用ISNULL的答案, @kanav正确地指出,COALESCE更贵。 CoALESCE评估了该小屋两次, 正如我在这里解释的 : < a href="https://stackoverflow.com/a/10669660/61305 > https://stackoverflow.com/a/10669660/61305
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