sql server - passing unquoted constants to functions like DATEPART does

Question

i would like to create a function which accepts a constant like the datepart function accepts yy/mm/dd/hh/

like:

select datepart(dd, getdate())

i would like to create my own function that accepts dd not char like 'dd'

i want

select MyFunc(dd, getdate())

and not

select MyFunc('dd', getdate())

Answer 1

You can't really constrain the input of a UDF to a small set of values (to the best of my knowledge).

I would recommend creating a tabe for your enumerated values - something like this:

CREATE TABLE MyEnumTable (DatePartID tinyint, DatePartValue char(2))
GO
INSERT MyEnumTable(DatePartID, DatePartValue)
SELECT 1, 'yy'
UNION
SELECT 2, 'mm'
UNION
SELECT 3, 'dd'
UNION
SELECT 4, 'hh'
GO

CREATE FUNCTION MyDatePart(@IntervalType tinyint)
RETURNS varchar(255)
AS
BEGIN
IF NOT EXISTS (SELECT * FROM MyEnumTable WHERE DatePartID = @IntervalType)
   RETURN 'Invalid IntervalType'

--Do your stuff
DECLARE @DatePartvalue char(2)
SELECT  @DatePartValue = DatePartValue
FROM    MyEnumTable
WHERE   DatePartID = @IntervalType

RETURN @DatePartValue
END

GO

--Check it out
SELECT dbo.MyDatePart(3), dbo.MyDatePart(12)

Of course, my example is oversimplified, but you get the idea.

Also, consider making the function a table-valued function for performance reasons, if you're planning on using the udf in set statements. I blogged about the performance implications of various function types here:

http://thehobt.blogspot.com/2009/02/scalar-functions-vs-table-valued.html

Answer 2

i would like to create my own function that accepts dd not char like 'dd'

I think you're out of luck on this. If you don't single-quote the characters, they'll be interpreted as a name--but you can't have defined them as a name anywhere if you want them to be used in the manner you propose.