Big hammer;
DECLARE @asd VARCHAR(20)
SET @asd = 'asdaaaadffa'
SELECT
Letter,
Occurrences = COUNT(*)
FROM (
SELECT Letter = SUBSTRING(@asd,n,1)
FROM (SELECT TOP(LEN(@asd)) n = ROW_NUMBER() OVER (ORDER BY (SELECT NULL))
FROM sys.all_columns) tally
) d
GROUP BY Letter
ORDER BY Occurrences DESC
For fast, accurate and documented assistance in answering your questions, please read this article.
Understanding and using APPLY, (I) and (II) Paul White
Hidden RBAR: Triangular Joins / The "Numbers" or "Tally" Table: What it is and how it replaces a loop Jeff Moden