CASE 语句除以另一个 CASE 语句给出值 0

Question

我正在根据每年的年份和月份计算比率。两个 CASE 语句都带来了正确的值，需要进行划分。但由于某种原因，我得到了 0 或 -100

;WITH cte_yoyComparison
AS
        (
SELECT  Year(EffectiveDate) AS EffectiveYear,
        Month(EffectiveDate) AS EffectiveMonth,
        SUM(Premium) as Premium 
FROM    Test_Plaza_ProductionReport 
WHERE YEAR(EffectiveDate) <> 2017
GROUP BY      Year(EffectiveDate),
                Month(EffectiveDate)            
            )
SELECT *, b.YearNum,b.MonthNum,

            ((case when b.YearNum=2016 and b.MonthNum=1 THEN Premium   end ))   /(case when b.YearNum = 2015 and b.MonthNum = 1 THEN Premium end)  as Ratio,
            ((((case when b.YearNum=2015 and b.MonthNum=1 THEN Premium   else 0 end))   /NULLIF((case when b.YearNum = 2016 and b.MonthNum = 1 THEN Premium   else 0 end),0))-1)*100 as Ratio1,
            case when b.YearNum=2015 and b.MonthNum=1 THEN Premium  else 0 end as CorrectValue,
            case when b.YearNum = 2016 and b.MonthNum = 1 THEN Premium else 0 end as CorrectValue1
 FROM tblCalendar b  
LEFT JOIN cte_yoyComparison a ON b.MonthNum=a.EffectiveMonth AND b.YearNum=a.EffectiveYear
WHERE b.YearNum<>2017
Order BY EffectiveYear desc,EffectiveMonth

Answer 1

您似乎想要比较 2 行而不是两列，但实际上是在比较 1 列和 2 个值。

((case when b.YearNum=2016 and b.MonthNum=1 THEN Premium   end ))   /(case when b.YearNum = 2015 and b.MonthNum = 1 THEN Premium end)  as Ratio,

将始终计算为 NULL / Premium 或 Premium / Null，因为 b.YearNum 不能是同一行中的 2 个值

((((case when b.YearNum=2015 and b.MonthNum=1 THEN Premium   else 0 end))   /NULLIF((case when b.YearNum = 2016 and b.MonthNum = 1 THEN Premium   else 0 end),0))-1)*100 as Ratio1,

将始终评估为 0 / (Premium -1) * 100 或 Premium / (-1 * 100)

您不是在比较行，而是在比较列，在这种情况下，同一列的值与 2 个不同的值。

您的意思是通过查看表a 而不是b 来评估分区的一侧？

只知道我可以在您的查询中看到什么，让您的 cte 保持原样，只需将您的查询从它更改为如下所示。请注意，我确实将您的 tblCalendar 从 a 更改为 c，因为对我来说日历以 c 开头。

SELECT *,
    ISNULL(a.Premium,0) / ISNULL(b.Premium,1) AS Ratio
FROM tblCalendar c  
    LEFT JOIN cte_yoyComparison a
    ON c.MonthNum=a.EffectiveMonth
    AND c.YearNum=a.EffectiveYear
    LEFT JOIN cte_yoyComparison b
    ON c.MonthNum=b.EffectiveMonth
    AND c.YearNum - 1 = b.EffectiveYear
WHERE
    c.YearNum<>2017
ORDER BY
    c.YearNum DESC
    ,c.MonthNum

它的作用是自我加入您的 cte 的另一个副本，但比 s.yearnum 少 1 年因为您使用的是外部联接，它将包括所有年份等，因此如果没有匹配项，您需要处理空值。之后无需比较任何日期，因为您可以保证表 a 比表 b 新 1 年。

Answer 2

我已将* 1.00 添加到您的分子中，这基本上会导致十进制转换。

;WITH cte_yoyComparison
AS
        (
SELECT  Year(EffectiveDate) AS EffectiveYear,
        Month(EffectiveDate) AS EffectiveMonth,
        SUM(Premium) as Premium 
FROM    Test_Plaza_ProductionReport 
WHERE YEAR(EffectiveDate) <> 2017
GROUP BY      Year(EffectiveDate),
                Month(EffectiveDate)            
            )
SELECT *, b.YearNum,b.MonthNum,

            ((case when b.YearNum=2016 and b.MonthNum=1 THEN Premium * 1.00  end ))   
                     /(case when b.YearNum = 2015 and b.MonthNum = 1 THEN Premium * 1.00 end)  as Ratio,
            ((((case when b.YearNum=2015 and b.MonthNum=1 THEN Premium * 1.00  else 0 * 1.00 end))   
                      /NULLIF((case when b.YearNum = 2016 and b.MonthNum = 1 THEN Premium * 1.00  else 0 * 1.00 end),0))-1)*100 as Ratio1,
            case when b.YearNum=2015 and b.MonthNum=1 THEN Premium * 1.00  else 0 * 1.00 end as CorrectValue,
            case when b.YearNum = 2016 and b.MonthNum = 1 THEN Premium * 1.00 else 0 * 1.00 end as CorrectValue1
 FROM tblCalendar b  
LEFT JOIN cte_yoyComparison a ON b.MonthNum=a.EffectiveMonth AND b.YearNum=a.EffectiveYear
WHERE b.YearNum<>2017
Order BY EffectiveYear desc,EffectiveMonth

现在查看您的数据，很明显为什么您没有获得所需的结果集。你的加入条件说

  b.MonthNum=a.EffectiveMonth AND b.YearNum=a.EffectiveYear

匹配月份和年份，但在 case 语句中，您使用不同年份的 Numerator 和 denominator。

您的 Case 语句与您的连接条件完全相反，因此 case 语句永远不会评估为 true，您将得到所有 NULLS。