Define a calculation field that returns text: Left ( YourField ; 1 )
now sort on that field.
Thank you for your reply.
I did that, but it doesn't sort that field correctly, the letters are not sorted: SSS, KKK, EEEE, etc. The sort result is a mixed order. I don't understand. Here's a (table view) of it:
make sure that text is specified as the return type for the calculation field.
That solved my problem. Thank you.