Sql - which letter of the alphabet is not in the names - sql

Question

I am a beginner at SQL
I need to find out which letter of the alphabet is not in a list of names as first character.
How can I do that?
To look for a specific letter I can use the LIKE operator. However I don't know what to use to look for the letter that is not in the list....
The query I have to find the different first letter of the emailadresses is:
select left(EmailAddress,1), count(left(EmailAddress,1)
from Customers
order by left(EmailAdress,1)
There is no e-mail adress that start with a U.
But which query I can use to get that result?

Answer 1

You'd need a table of all letters. You can use an ad hoc derived table using UNION ALL and FROM-less SELECTs. One method then is to use a correlated subquery and NOT EXISTS to check for letters not in the first position of any e-mail address.
SELECT l.letter
FROM (SELECT 'a' letter
UNION ALL
SELECT 'b' letter
...
SELECT 'y' letter
UNION ALL
SELECT 'z' letter) l
WHERE NOT EXISTS (SELECT *
FROM customers c
WHERE left(lower(emailaddress), 1) = l.letter);

Answer 2

One could left join to letters and filter the unmatched.
The example below uses a recursive CTE to generate the letters.
WITH RCTE_LETTERS AS
(
SELECT CHAR(ASCII('a')) AS Letter, ASCII('a') AS Code
UNION ALL
SELECT CHAR(code+1), code+1
FROM RCTE_LETTERS
WHERE code < ASCII('z')
)
, CTE_FIRST_LETTERS AS
(
SELECT DISTINCT
LOWER(LEFT(EmailAddress,1)) AS FirstLetter
FROM Customers
)
SELECT l.Letter
FROM RCTE_LETTERS AS l
LEFT JOIN CTE_FIRST_LETTERS AS fl
ON fl.FirstLetter = l.Letter
WHERE fl.FirstLetter IS NULL
ORDER BY l.Letter;
Or use an EXCEPT
WITH RCTE_LETTERS AS
(
SELECT CHAR(ASCII('a')) AS Letter, ASCII('a') AS Code
UNION ALL
SELECT CHAR(code+1), code+1
FROM RCTE_LETTERS
WHERE code < ASCII('z')
)
, CTE_FIRST_LETTERS AS
(
SELECT DISTINCT
LOWER(LEFT(EmailAddress,1)) AS FirstLetter
FROM Customers
)
SELECT Letter FROM RCTE_LETTERS
EXCEPT
SELECT FirstLetter FROM CTE_FIRST_LETTERS
ORDER BY Letter

Answer 3

USE AdventureWorks2014
GO
SELECT left(e.EmailAddress,1) AS Letter, COUNT(left(e.EmailAddress,1)) AS CountEmailLetter
FROM Person.Person p INNER JOIN person.EmailAddress e ON p.BusinessEntityID = e.BusinessEntityID
WHERE left(e.EmailAddress,1) NOT IN
(
SELECT Char(number+65)
FROM master.dbo.spt_values
WHERE name IS NULL AND number < 26
)
GROUP BY left(e.EmailAddress,1)
ORDER BY left(e.EmailAddress,1)