That is because this is a bad idea. Once you start "auto incrementing" characters you open up a whole possibility of bad things. What happens when you start getting into 3 and 4 letter words?

You already have a unique value per row. Why can't you use that? Or if you really need to use somenum why not cast it to a varchar add an underscore and then use row_number?

Something like this maybe?

select uniquenum, Somenum, CAST(Somenum varchar(10)) + '_' + cast(Suffix as varchar(5)) as SomeNum_PlusRowNum

from

(

SELECT UniqueNum

,Somenum

, ROW_NUMBER() OVER (PARTITION BY SomeNum ORDER BY UniqueNum ASC) AS Suffix

FROM #temp_Alpha_num

)

ORDER BY uniquenum, Somenum ASC;