Conditionally setting a variable in SQL

Question

I'm trying to do this:

• If the Day parameter is the current day, then set @EndDate to be yesterday instead
• If the Day parameter is in the future, set @EndDate to yesterday.

I've tried a few different approaches including two down below. I'm fairly new to programming so odds are I'm missing something fairly simple. Basically, I am trying to set @EndDate conditionally, depending on what @Day is set to be.

DECLARE @Day DATETIME

SET @Day = '09/2/17 12:50'  
SET @Day = DATEADD(dd, DATEDIFF(dd, 0, @Day), 0)

DECLARE @Enddate DATETIME

SET @Enddate = CASE @Day  
                  WHEN @Day < GETDATE() THEN GETDATE() - 1
               END

--SET @Enddate = @Day  
--     WHERE @Day < GETDATE()  
--SET @Enddate = GETDATE()-1    
--     WHERE@Day >= GETDATE()

Thanks

Answer 1

You are mixing the two possible ways to write a case expression...

You can either use

CASE @day 
   WHEN GETDATE() THEN 'x' 
   WHEN DATEADD(DAY, 1, GETDATE() THEN 'y' 
END

or then you can use:

CASE 
   WHEN @day = GETDATE() THEN 'x' 
   WHEN @day > GETDATE() THEN 'y' 
END

This is the correct way:

SET @Enddate = CASE 
                  WHEN @Day < GETDATE() 
                     THEN DATEADD(DAY, -1, GETDATE())
                     ELSE GETDATE()
               END

Answer 2

For clarification, variable @yesterday added as DATE without time

Declare @Day datetime

Set @Day = '09/02/17 12:50'
SET @Day = DATEADD(dd, DATEDIFF(dd, 0, @Day), 0)

Declare @Enddate Datetime

Declare @yesterday as date
SET @yesterday=dateadd(day,-1,getdate())

Set @Enddate = Case 
When @day<  @yesterday Then @day else @yesterday  End   

Any values older than yesterday remains as is, other case sets yesterday