SELECT
SUBSTR(str, nums.n, 1) AS letter,
COUNT(*) AS count
FROM
(SELECT 'your_string' AS str) AS str_table
CROSS JOIN
(SELECT ROW_NUMBER() OVER () AS n FROM (VALUES (1), (2), (3), (4), (5), (6), (7), (8), (9), (10)) AS numbers(n)) AS nums
WHERE
nums.n <;= LENGTH(str_table.str)
GROUP BY
SUBSTR(str, nums.n, 1)
ORDER BY
letter;