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Creating a Function

In Transact-SQL, the primary formula of creating a function is:

CREATE FUNCTION FunctionName()

For a function to be useful, it must produce a result. This is also said that the function returns a result or a value. When creating a function, you must specify the type of value that the function would return. To provide this information, after the name of the function, type the RETURNSkeyword followed by a definition for a data type. Here is a simple example:
CREATE FUNCTION Addition()
RETURNS Decimal(6,3)
After specifying the type of value that the function would return, you can create a body for the function. The body of a function starts with the BEGIN and ends with the END keywords. Here is an example:
CREATE FUNCTION Addition()
RETURNS Decimal(6,3)
BEGIN

END
Optionally, you can type the AS keyword before the BEGIN keyword:
CREATE FUNCTION Addition()
RETURNS Decimal(6,3)
AS
BEGIN

END
Between the BEGIN and END keywords, which is the section that represents the body of the function, you can define the assignment the function must perform. After performing this assignment, just before the END keyword, you must specify the value that the function returns. This is done by typing the RETURN keyword followed by an expression. A sample formula is:
CREATE FUNCTION Addition()
RETURNS Decimal(6,3)
AS
BEGIN
    RETURN Expression
END
Here is an example
CREATE FUNCTION GetFullName()
RETURNS varchar(100)
AS
BEGIN
 RETURN 'Doe, John'
END
Here is another example:
CREATE FUNCTION CalculateWeeklySalary()
RETURNS Decimal(8, 2)
AS
BEGIN
    RETURN 880.44
END;
GO
Function Calling
After a function has been created, you can use the value it returns. Using a function is also referred to as calling it. To call a function, you must qualify its name. To do this, type the name of the database in which it was created, followed by the period operator, followed by dbo, followed by the period operator, followed by the name of the function, and its parentheses. The formula to use is:
DatabaseName.dbo.FunctionName()
Because a function returns a value, you can use that value as you see fit. For example, you can use either PRINT or SELECT to display the function's value in a query window. Here is an example that calls the above Addition() function:
PRINT Exercise.dbo.GetFullName();
As an alternative, to call a function, in the Object Explorer, right-click its name, position the mouse on Script Function As, SELECT To, and click New Query Editor Window.
Renaming a Function
To rename a function, in the Object Explorer, right-click it and click Rename. Type the desired new name and press Enter.
Modifying a Function
As mentioned already, in the body of the function, you define what the function is supposed to take care of. As a minimum, a function can return a simple number, typed on the right side of the RETURN keyword. Here is an example:
CREATE FUNCTION Addition()
RETURNS int
BEGIN
    RETURN 1
END
You can also declare new variables in the body of the function to help in carrying the assignment. A variable declared in the body of a function is referred to as a local variable. Once such a variable has been declared, it can be used like any other variable. Here is an example:
CREATE FUNCTION Addition()
RETURNS int
BEGIN
    DECLARE @Number1 int
    SET @Number1 = 588
    RETURN @Number1 + 1450
END
A Parameterized Function
To create a function that takes a parameter, specify a name and the type of value of the parameter(s) in its parentheses. Here is an example:
CREATE FUNCTION Addition(@Number1 Decimal(6,2))
When a function takes a parameter, in the body of the function, you can use the parameter as if you knew its value, as long as you respect the type of that value. Here is an example:
CREATE FUNCTION Addition(@Number1 Decimal(6,2))
RETURNS Decimal(6,2)
BEGIN
    RETURN @Number1 + 1450
END
Calling a Parameterized Function
When you call a function that takes one parameter, you must supply a value for that argument. To do this, type the value of the parameter in the parentheses of the function. Here is an example:
A Function With Various Arguments
Instead of only one parameter, you can also create a function that takes more than one parameter. In this case, separate the arguments in the parentheses of the function with a comma. Here is an example:
CREATE FUNCTION Addition(@Number1 Decimal(6,2), @Number2 Decimal(6,2))
Once again, in the body of the function, you can use the parameters as if you already knew their value. You can also declare local variables and involve them with parameters as you see fit. Here is an example:
CREATE FUNCTION Addition(@Number1 Decimal(6,2),
    @Number2 Decimal(6,2))
RETURNS Decimal(6,2)
BEGIN
    DECLARE @Result Decimal(6,2)
    SET @Result = @Number1 + @Number2
    RETURN @Result
END;
GO
When calling a function that takes more than one parameter, in the parentheses of the function, provide a value for each parameter, in the exact order they appear in the parentheses of the function. Here is an example:
PRINT Variables1.dbo.Addition(1450, 228);
You can also pass the names of already declared and initialized variables. Here is an example that calls the above function:
DECLARE @Nbr1 Decimal(6,2),
        @Nbr2 Decimal(6,2)
SET @Nbr1 = 4268.55
SET @Nbr2 =26.83
SELECT @Nbr1 As First,
       @Nbr2 As Second,
       Variables1.dbo.Addition(@Nbr1, @Nbr2) AS Result
This would produce:
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