如何返回0(而不是NULL)的count(*)

Question

我有这段代码:

SELECT Project, Financial_Year, COUNT(*) AS HighRiskCount
INTO #HighRisk 
FROM #TempRisk1
WHERE Risk_1 = 3
GROUP BY Project, Financial_Year

当计数为零时,它不会返回任何行.如何在HighRiskCount设置为0的情况下显示这些行？

Answer 1

当行计数为0时,您无法从表中选择值.在哪里可以获得不存在的行的值？

为此,您必须有另一个表来定义有效值Project和Financial_Year值列表.然后,您将从此表中进行选择,left join在现有表上执行a ,然后进行分组.

像这样的东西:

SELECT l.Project, l.Financial_Year, COUNT(t.Project) AS HighRiskCount
INTO #HighRisk 
FROM MasterRiskList l
left join #TempRisk1 t on t.Project = l.Project and t.Financial_Year = l.Financial_Year
WHERE t.Risk_1 = 3
GROUP BY l.Project, l.Financial_Year

Answer 2

将您的SELECT查询包装在一个ISNULL：

SELECT ISNULL((SELECT Project, Financial_Year, COUNT(*) AS hrc
INTO #HighRisk 
FROM #TempRisk1
WHERE Risk_1 = 3
GROUP BY Project, Financial_Year),0) AS HighRiskCount

如果您SELECT返回一个数字，它将通过。如果它返回NULL，0将通过。

Answer 3

假设您有您的“项目”和“财务年”，其中​​ Risk_1 与 3 不同，而这些是您打算包括的。

SELECT Project, Financial_Year, SUM(CASE WHEN RISK_1 = 3 THEN 1 ELSE 0 END) AS HighRiskCount
INTO #HighRisk 
FROM #TempRisk1
GROUP BY Project, Financial_Year

注意我删除了 where 部分。

顺便说一下，您当前的查询没有返回 null，它没有返回任何行。

Answer 4

MSDN - ISNULL 函数

SELECT Project, Financial_Year, ISNULL(COUNT(*), 0) AS HighRiskCount
INTO #HighRisk 
FROM #TempRisk1
WHERE Risk_1 = 3
GROUP BY Project, Financial_Year