For ascii, lower case a to z is 97 to 122; upper case is 65 to 90 (decimal). A table would be easier I think, if you had to be concerned about not using the same character twice. In that case, a yes/no field could also be used to flag the letters as having been used. Or are there only 4 letters involved and you're trying to create a list of unique combinations that start using the letter part over and over again when you get to D?
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.