Scalar-Valued User Defined Function in SQL SERVER

We will look into the Scalar-Valued User Defined Function

The syntax for Scalar-Valued is as follows
CREATE FUNCTION [ schema_name. ] function_name ( [ { @parameter_name [ AS ][ type_schema_name. ] parameter_data_type     [ = default ] [ READONLY ] }     [ ,...n ]   ] )
RETURNS return_data_type     [ WITH [ ,...n ] ]     
[ AS ]     
BEGIN         
function_body         
RETURN scalar_expression     
END [ ; ]


The below example shows how a function returns the phone number in US Phone format (XXX)XXX-XXXX

CREATE FUNCTION USFORMATPHONE(@PhoneNumber varchar(25))
RETURNS Varchar(50) AS
BEGIN
DECLARE @phone varchar(50)
IF(LEN(@PhoneNumber) = 10)
SET @phone = '('+ SUBSTRING(@PhoneNumber,1,3) + ')' + SUBSTRING(@PhoneNumber,4,3) + '-' + SUBSTRING(@PhoneNumber,7,4)
ELSE
SET @phone = 'Invalid Phone Number'
Return @phone
END

The usage of the same is as follows

SELECT EMPLOYEE.DBO.USFORMATPHONE ('1234567890')