Increment a string

It's good to create a serial number to tickets, or another serie from data type character.

declare @litere nvarchar(3)
declare @litera1 char(1)
declare @litera2 char(1)

set @litere='AQ'

select @litera1=substring(@litere,2,1)
select @litera2=substring(@litere,1,1)

if @litera1='Z'
begin
set @litera1='A'
set @litera2=char(ascii(substring(@litere,1,1))+1)
end

else
set @litera1=char(ascii(substring(@litere,2,1))+1)

select @litera1, @litera2
set @litere=@litera2+@litera1
select @litere