Let's start from two reciprocally related functions - ASCII and CHAR.
The ASCII function returns an ASCII code value of the leftmost character of a character expression, being a function argument.
Here is an example of the way to determine how many different letters which the names of the ships in the Ships table start from are:
SELECT COUNT(DISTINCT ASCII(name)) FROM Ships
The result is 11. To find out what these letters are, we can use CHAR function that returns a character by a known ASCII code value (from 0 to 255):
SELECT DISTINCT CHAR(ASCII(name)) FROM Ships ORDER BY 1