How to Write a function in sql server

CREATE FUNCTION whichContinent
(@Country nvarchar(15))
RETURNS varchar(30)
AS
BEGIN
declare @Return varchar(30)
select @return = case @Country
when 'Argentina' then 'South America'
when 'Belgium' then 'Europe'
when 'Brazil' then 'South America'
when 'Canada' then 'North America'
when 'Denmark' then 'Europe'
when 'Finland' then 'Europe'
when 'France' then 'Europe'
else 'Unknown'
end

return @return
end

o/p:
print dbo.WhichContinent('France')

Europe

select dbo.WhichContinent(Customers.Country), customers.* 
from customers

create table test
(Country varchar(15),
Continent as (dbo.WhichContinent(Country)))

insert into test (country) 
values ('USA')

select * from test

- - - - - - - - - - - - - - - - - - - - - -
Country          Continent
---------------  --------------------------

USA              North America