how to make nvarchar id auto incerement (1) function in sqlserver

create function idincr() returns nvarchar(50)

as begin

declare @id int, @cid nvarchar(50), @did int

set @id=(select count(*) from tblcust)

if(@id>0)

begin

set @did= (select max(cast(substring(custid,2,len(custid))as int))from tblcust)+1;

set @cid='A'+cast(@did as nvarchar(50))

end

else 

set @cid='A1'

return @cid

end

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select dbo.idincr()

Go