https://www.youtube.com/watch?v=iKQB4WtN-C4

DROP TABLE IF EXISTS Combinations
;
CREATE TABLE Combinations
(
  Word varchar(100)
)
;
INSERT INTO Combinations
VALUES
('A'),
('B'),
('C'),
('D'),
('-'), 
('Apple'),
('Ball'),
('Cat'),
('Dog'),
('-5515'),
('-5255'),
('Ant')

SELECT * FROM Combinations

WITH
LEFT()
REGEXP

-- Approach 1

WITH cte_alphabet as (
select 
	*
from Combinations
where word REGEXP '^.

SUBQUERY
```sql
-- Approach 2

select * 
from combinations a 
left join (select * from combinations where length(word)>1) b ON a.word = left(b.word,1)
where b.word is not null
```)
,cte_words as (
select 
	*
	,left(word,1) as first
from combinations
where word NOT REGEXP '^.

[[SUBQUERY]]
{{CODE_BLOCK_2}}
)
select c.word as Alphabet,cw.word as Word from cte_alphabet c join cte_words cw on c.word = cw.first
order by word

[[SUBQUERY]]