如何翻译嵌套的 CASE 表达式以理解逻辑？答案

【问题标题】：How to translate nested CASE expression in order to understand the logic?如何翻译嵌套的 CASE 表达式以理解逻辑？
【发布时间】：2016-12-30 00:59:53
【问题描述】：

我有一个由其他人创建的存储过程。在WHERE 子句中，有一个 CASE 表达式，我很难理解：

 DECLARE
        @DateFrom datetime = '01-01-2016',
        @DateTo datetime = '12-31-2016'
        @EffDateFrom datetime = NULL,
        @EffDateTo datetime = NULL, 
    /*    SOME  SELECT statement here   */
WHERE      
             CASE WHEN @EffDateFrom IS NULL THEN 1
                            ELSE CASE WHEN dateDiff(d, '01-01-2016', dbo.tblQuotes.EffectiveDate) >= 0 
                                    AND dateDiff(d, EOMONTH (GETDATE()), dbo.tblQuotes.EffectiveDate) <= 0 Then 1 
                                  else 0 
                                  end
                            END = 1

--------------/* This is where I am confused */--------------------------------

                        AND CASE WHEN @DateFrom IS NULL THEN 1 ELSE
                            CASE WHEN INV.InvoiceDate > INV.EffectiveDate THEN 
                                CASE WHEN dateDiff(d, '01-01-2016', INV.InvoiceDate) >= 0 
                                    AND dateDiff(d, '12-31-2016', INV.InvoiceDate) <= 0 Then 1 else 0 end
                            ELSE
                                CASE WHEN dateDiff(d, '01-01-2016', INV.EffectiveDate) >= 0 
                                    AND dateDiff(d, '12-31-2016', INV.EffectiveDate) <= 0 Then 1 else 0 end
                            END 
                            END = 1

所以我们在这里说的是：

当参数@DateFrom为null时，使用参数@EffDateFrom，对吗？

但如果它不为空，那么 1.检查InvoiceDate是否大于EffectiveDate，如果是-那么 2.检查01-01-2016和InvoiceDate之间的天数是否大于或等于0AND如果12-31-2016和InvoiceDate之间的天数小于或等于0则为1！！什么是1？意味着记录是有效的？记录将在一个表中？对吗？

ELSE 0 表示它不会选择记录，对吗？

在那之后我很困惑。更新理解（如果正确）：

  CASE WHEN @DateFrom IS NULL THEN 1 ELSE

                            CASE WHEN INV.InvoiceDate > INV.EffectiveDate THEN 
-----------------------------/*then check the below conditions and if its 1 then display the record if its 0 then do NOT */
                                        CASE WHEN   dateDiff(d, '01-01-2016', INV.InvoiceDate) >= 0 
                                                    AND dateDiff(d, '12-31-2016', INV.InvoiceDate) <= 0 THEN 1 ELSE 0 
                                        END
----------------------------/* and this statement will only be working  if parameter @EffDateFrom is not null. Correct? */
                            ELSE
                                CASE WHEN dateDiff(d, '01-01-2016', INV.EffectiveDate) >= 0 
                                    AND dateDiff(d, '12-31-2016', INV.EffectiveDate) <= 0 Then 1 else 0 end
                            END 
                            END = 1

【问题讨论】：

标签： sql-server tsql case datediff

【解决方案1】：

这是（你问的部分的）逻辑：

IF (@DateFrom IS NULL) 
    OR ((INV.InvoiceDate > INV.EffectiveDate) AND (dateDiff(d, '01-01-2016', INV.InvoiceDate) >= 0) AND (dateDiff(d, '12-31-2016', INV.InvoiceDate) <= 0) )
    OR ((INV.InvoiceDate <= INV.EffectiveDate) AND (dateDiff(d, '01-01-2016', INV.EffectiveDate) >= 0) AND (dateDiff(d, '12-31-2016', INV.EffectiveDate) <= 0)) 
    OR ((INV.InvoiceDate IS NULL) AND (dateDiff(d, '01-01-2016', INV.EffectiveDate) >= 0) AND (dateDiff(d, '12-31-2016', INV.EffectiveDate) <= 0))
    OR ((INV.EffectiveDate IS NULL) AND (dateDiff(d, '01-01-2016', INV.EffectiveDate) >= 0) AND (dateDiff(d, '12-31-2016', INV.EffectiveDate) <= 0))
THEN TRUE
ELSE FALSE

整个声明：

IF (@EffDateFrom IS NULL)
    OR ((dateDiff(d, '01-01-2016', dbo.tblQuotes.EffectiveDate) >= 0 )  AND (dateDiff(d, EOMONTH (GETDATE()), dbo.tblQuotes.EffectiveDate) <= 0) )
THEN TRUE
ELSE FALSE

AND

IF (@DateFrom IS NULL) 
    OR ((INV.InvoiceDate > INV.EffectiveDate) AND (dateDiff(d, '01-01-2016', INV.InvoiceDate) >= 0) AND (dateDiff(d, '12-31-2016', INV.InvoiceDate) <= 0) )
    OR ((INV.InvoiceDate <= INV.EffectiveDate) AND (dateDiff(d, '01-01-2016', INV.EffectiveDate) >= 0) AND (dateDiff(d, '12-31-2016', INV.EffectiveDate) <= 0)) 
    OR ((INV.InvoiceDate IS NULL) AND (dateDiff(d, '01-01-2016', INV.EffectiveDate) >= 0) AND (dateDiff(d, '12-31-2016', INV.EffectiveDate) <= 0))
    OR ((INV.EffectiveDate IS NULL) AND (dateDiff(d, '01-01-2016', INV.EffectiveDate) >= 0) AND (dateDiff(d, '12-31-2016', INV.EffectiveDate) <= 0))
THEN TRUE
ELSE FALSE

【讨论】：

【解决方案2】：

如果不使用case语句，你所困惑的部分可以解释为以下内容：

WHERE 
          @DateFrom IS NULL 

OR   (    @DateFrom IS NOT NULL 
      AND INV.InvoiceDate > INV.EffectiveDate  
      AND dateDiff(d, '01-01-2016', INV.InvoiceDate) >= 0  
      AND dateDiff(d, '12-31-2016', INV.InvoiceDate) <= 0
      )

OR   (    @DateFrom IS NOT NULL 
      AND dateDiff(d, '01-01-2016', INV.EffectiveDate) >= 0 
      AND dateDiff(d, '12-31-2016', INV.EffectiveDate) <= 0 
     )

【讨论】：

啊哈！！知道了！非常感谢#M.Ali