How to use if condition in a select query? - sql-server

Question

I have a start date and end date column in my table and I get a start date and end date as input. I need to set START date as TBL.StartDate if it is after INPUT Start Date and I need to set END date as TBL.EndDate if it is before INPUT End Date. Finally count the days between START and END.
I'm trying to use if statement but I fail when I try to use if in select query.
I'm not sure if this is a correct way of solving this problem.
Declare #Start Date;
Declare #End Date;
Declare #INPUT_StartDate Date;
Declare #INPUT_EndDate Date;
set #INPUT_StartDate = '2019-01-01'; --user input
set #INPUT_EndDate = '2019-04-20'; --user input
select
TBL.StartDate,
TBL.EndDate,
#INPUT_StartDate,
#INPUT_EndDate,
if(TBL.StartDate < INPUT_StartDate) set #Start = TBL.STARTDate;
else set #Start = INPUT_StartDate;
if(TBL.EndDate < INPUT_EndDate) set #End = INPUT_EndDate;
else set #End = TBL.EndDate;
#DateDiff(day, #Start, #End) as COUNT
from TBL
I'm expecting that this comparison happens, but i guess i'm using if statement in between select query is not allowed?
Actual results are quite a few syntax errors near if.

Answer 1

You can make sure of case statement which is same as if but used in case statement. if you want to use if in select list(there is IIF if your sql server is 2012+). If is mostly used in conditional logic.
I removed #start and #end date variable which might not be required based on the logic you are using.
select '2019-03-20' as startdate, '2019-04-20' as ENddate into #temp union all
select '2018-02-01' as startdate, '2019-03-20' as ENddate
Declare #Start Date;
Declare #End Date;
Declare #INPUT_StartDate Date;
Declare #INPUT_EndDate Date;
set #INPUT_StartDate = '2019-01-01'; --user input
set #INPUT_EndDate = '2019-04-20'; --user input
select
TBL.StartDate,
TBL.EndDate,
#INPUT_StartDate INPUT_StartDate,
#INPUT_EndDate INPUT_EndDate,
datediff(day, case when tbl.StartDate < #INPUT_StartDate then TBL.STARTDate
else #INPUT_StartDate end,
case when TBL.EndDate < #INPUT_EndDate then #INPUT_EndDate
else TBL.EndDate end) date_diff
from #temp tbl
Output:
Startdate Enddate Input_startdate input_enddate date_diff
2019-03-20 2019-04-20 2019-01-01 2019-04-20 109
2018-02-01 2019-03-20 2019-01-01 2019-04-20 443

Answer 2

SQL Server doesn't have least() or greatest(). And SQL Server doesn't allow you to mix selecting columns with setting variables.
I recommend setting the values in a "subquery", in this case, using apply:
select t.StartDate, t.EndDate,
#INPUT_StartDate, #INPUT_EndDate,
DateDiff(day, v.dateStart, v.dateEnd) as num_days
from tbl t cross apply
(values (case when t.StartDate < #INPUT_StartDate
then t.STARTDate
else #INPUT_StartDate
end,
case when t.EndDate < #INPUT_EndDate
then INPUT_EndDate
else #TBL.EndDate
end
)
) v(startDate, endDate);

Answer 3

declare #StartDate datetime,#msg varchar(100),#Diff int
declare #EndDate datetime
declare #TodayDate datetime
set #StartDate = '2019-04-23' ---Define Start Date-----
set #EndDate = '2019-04-30' ---Define End Date-----
set #TodayDate = GETDATE()
if(#StartDate < #TodayDate)
begin
set #msg = 'Date is Smaller than today date'
select #msg
end
else
begin
select #Diff = DATEDIFF(day,#StartDate,#EndDate)
set #msg = #Diff
select #msg as Days_Differance
end