How to translate nested CASE expression in order to understand the logic?

Question

I have a stored procedure that was created by someone else. In the WHERE clause, there is a CASE expression that I am having a hard time to understand:

 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

So what we saying here is:

When Parameter @DateFrom is null, then use parameter @EffDateFrom, is that right?

But if its not null, then 1.check if InvoiceDate greater than EffectiveDate, and if it is - then 2.check if the number of days between 01-01-2016 and InvoiceDate is greater or equal to 0 AND if the number of days between 12-31-2016 and InvoiceDate less or equal to 0 then 1!! what is 1? meaning the record is valid? The record will be in a table? Correct?

And ELSE 0 meaning it will NOT pick the record, correct?

After that I am confused. Updated the understanding ( if its correct):

  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

Answer 1

This is the logic (of the part you asked):

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

The whole statement:

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

Answer 2

The Part you are confused about can be interpreted to the following without the use of case statement:

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 
     )