Unable to use variable in DATEADD(#pinterval, #pinterval, #DayPlus) - tsql

Question

DECLARE #pinterval INT = 1, #DayPlus DATETIME = '2016-07-01', #datepart VARCHAR(20) = 'MONTH'
SET #DayPlus = DATEADD(#datepart, #pinterval, #DayPlus)
SELECT #DayPlus
Do we have any alternative to accomplish task ? I have to do it in loop so I can't define it every time based on interval value. Only date part is not acceptable as variable because if I use as
DATEADD(MONTH, #pinterval, #DayPlus)
then it's working fine. Not sure but I can understand the issue but I am seeking for the quick solution. I have visited the web but didn't get exact solution.

Answer 1

It's not possible to substitute a variable for the first argument to DATEADD1, since what is wanted here is a name, not a string.
About the best you can do is a CASE expression:
SET #DayPlus =
CASE #datepart
WHEN 'MONTH' THEN DATEADD(MONTH, #pinterval, #DayPlus)
WHEN 'YEAR' THEN DATEADD(YEAR, #pinterval, #DayPlus)
WHEN 'DAY' THEN DATEADD(DAY, #pinterval, #DayPlus)
--TODO - add all parts you might wish to use
END
1This is even stated in the documentation:
User-defined variable equivalents are not valid.

Answer 2

if this is only the way then I have to repeat this for more than 10times
No, use CROSS APPLY
#Shnugo it sounds good, can you please help me by placing the complete code as an answer. Please !!
About CROSS APPLY: generate something like a variable dynamically
CREATE TABLE dbo.Test(ID INT, SomeDate DATE);
GO
INSERT INTO dbo.Test VALUES(1,{d'2017-01-01'}),(2,{d'2017-02-02'})
GO
DECLARE #Intervall VARCHAR(100)='DAY';
DECLARE #Count INT=1
SELECT dbo.Test.*
,t.AddedDate
FROM dbo.Test
CROSS APPLY(SELECT CASE #Intervall WHEN 'MONTH' THEN DATEADD(MONTH,#Count,SomeDate)
WHEN 'DAY' THEN DATEADD(DAY,#Count,SomeDate)
ELSE SomeDate END AS AddedDate) AS t;
GO
DROP TABLE dbo.Test;

Answer 3

You could use dynamic SQL:
DECLARE #pinterval INT = 1,
#DayPlus DATETIME = '2016-07-01',
#datepart NVARCHAR(20) = 'MONTH'
DECLARE #sql NVARCHAR(MAX) = N'SET #DayPlus = DATEADD('+#datepart+', #pinterval, #DayPlus)',
#params NVARCHAR(MAX) = N'#DayPlus DATETIME OUTPUT, #pinterval INT'
EXEC sp_executesql #sql, #params, #pinterval = #pinterval, #DayPlus = #DayPlus OUTPUT
SELECT #DayPlus
sp_executesql is used for 2 reasons:
to minimize SQL injections risk, it will not eliminate it because of #datepart
it will make sure your dynamic query's plan is going to be cached - which might be beneficial with simple queries (where query time vs compilation time matters)