How to query duplicates results MySQL - mysql

Question

While every question I found is talking about removing duplicates I need these duplicates.
Let's say my database is
+-------+-----------+
| ID | letter |
+-------+-----------+
| 1 | A |
| 2 | B |
| 4 | Z |
+-------+-----------+
I need to query a person name so let say the name is "ABA" when I query like this
select * from letters where letter = 'A' or letter = 'B' or letter = 'A'
My result will be
+-------+-----------+
| ID | letter |
+-------+-----------+
| 1 | A |
| 2 | B |
+-------+-----------+
I want the output will include the 3rd letter as a separate row.
+-------+-----------+
| ID | letter |
+-------+-----------+
| 1 | A |
| 2 | B |
| 3 | A |
+-------+-----------+
Maybe I don't know the right term but I didn't find even one answer that give me half a solution.
there is one entry but can I can the entry again? if I query for "nina" get the full name and not just "nia"

Answer 1

Original answer (and recommended approach)
Use a recursive query to convert the name into rows with one letter each. Then join with your letters table.
with recursive word_letters (word, pos, letter) as
(
select #name, 1, substr(#name, 1, 1)
union all
select word, pos + 1, substr(word, pos + 1, 1)
from word_letters
where pos < length(word)
)
select letters.*
from word_letters
join letters on letters.letter = word_letters.letter
order by word_letters.pos;
Demo: https://dbfiddle.uk/?rdbms=mysql_8.0&fiddle=6547c21d2dd9223270615047d46d9783
UPDATE: Workaround for old MySQL versions
Build a table of positions (numbers) large enough to cover the longest word. Then join the word in order to get the position for each letter in it. Then join your other table.
select letters.*
from
(
select hundreds.digit * 100 + tens.digit * 10 + units.digit + 1 as pos
from (select 0 as digit union select 1 union select 2 union select 3 union select 4 union select 5 union select 6 union select 7 union select 8 union select 9) units
cross join (select 0 as digit union select 1 union select 2 union select 3 union select 4 union select 5 union select 6 union select 7 union select 8 union select 9) tens
cross join (select 0 as digit union select 1 union select 2 union select 3 union select 4 union select 5 union select 6 union select 7 union select 8 union select 9) hundreds
) positions
join (select #name as word) w on length(w.word) >= positions.pos
join letters on letters.letter = substr(w.word, positions.pos, 1)
order by positions.pos;
Demo: https://dbfiddle.uk/?rdbms=mysql_5.7&fiddle=db3762d5705ce5eb77e628c4d4058485

Answer 2

You can do it by using any of the below queries. For your reference visit the link below to test the output.
http://sqlfiddle.com/#!9/c5e9f3/1
The table created and the populated data is as below
create table tbl(ID int,letter varchar(2));
insert into tbl(ID,letter) values(1,'A');
insert into tbl(ID,letter) values(2,'B');
insert into tbl(ID,letter) values(3,'A');
insert into tbl(ID,letter) values(4,'Z');
insert into tbl(ID,letter) values(5,'C');
and now the query
select * from tbl where letter in ('A','B');
or
select * from tbl where letter ='A' or letter='B';