Define a function and use it in a SQL-Query

Question

I was tring to create a function and use it in my little Query. I did not do this before and I do not really know to do it.

Is it simply the same like any other languages? like

create function testFunction()...
begin
..
..
end

and then for example :

Select testFunction() 

and it should work ?

CREATE FUNCTION dbo.ISOweek (@DATE datetime)  
RETURNS int  
WITH EXECUTE AS CALLER  
AS  
BEGIN  
     DECLARE @ISOweek int
     SET @ISOweek= DATEPART(wk,@DATE)+1  
          -DATEPART(wk,CAST(DATEPART(yy,@DATE) as CHAR(4))+'0104')
--Special cases: Jan 1-3 may belong to the previous year  
     IF (@ISOweek=0)   
          SET @ISOweek=dbo.ISOweek(CAST(DATEPART(yy,@DATE)-1   
               AS CHAR(4))+'12'+ CAST(24+DATEPART(DAY,@DATE) AS CHAR(2)))+1
--Special case: Dec 29-31 may belong to the next year  
     IF ((DATEPART(mm,@DATE)=12) AND   
          ((DATEPART(dd,@DATE)-DATEPART(dw,@DATE))>= 28))  
          SET @ISOweek=1
     RETURN(@ISOweek)
END;  
GO  
SET DATEFIRST 1
SELECT dbo.ISOweek(CONVERT(DATETIME,'12/26/2004',101)) AS 'ISO Week'

Answer 1

Yes, SQL functions are easy to create. But you have to understand the 3 different types of functions in SQL:
1) Scalar functions:
-- return a single value.
2) Table based functions:
-- returns a Table.
3) Aggregate function:
returns a single value (but the function looped through a window set).

creating a function in MS SQL Server 2012 by using template:

There some great reference resources for starting creating SQL functions: an example of User Defined Scalar Function (from dotnettricks):

        --Create function to get emp full name 
        Create function fnGetEmpFullName
        (
          @FirstName varchar(50),
          @LastName varchar(50)
        )
        returns varchar(101)
        AS
        BEGIN 
          return (Select @FirstName + ' '+ @LastName);
        END

Microsoft reference, explanation and examples in this link
informit article on User Defined Functions in this link2
and finally from SQL Team website, an introduction to User Defined Functions