Storing numbers that will be also used as digits

Your best solution for this is undoubtedly the use of GENERATED columns (also known as VIRTUAL or COMPUTED columns).

You mention MariaDB - the documentation is available here. The core bit (the syntax) is here:

<type> [GENERATED ALWAYS] AS ( <expression> ) [VIRTUAL | PERSISTENT | STORED] [UNIQUE] [UNIQUE KEY] [COMMENT <text>] 

So, for MariaDB, I constructed the following table here (fiddle):

CREATE TABLE the_code_1 ( code CHAR(8), the_code_location CHAR(1) GENERATED ALWAYS AS (SUBSTR(code, 3, 1)) VIRTUAL COMMENT '3rd character' ); 

Then I INSERTED some data:

INSERT INTO the_code_1 (code) VALUES ('12345678'); INSERT INTO the_code_1 (code) VALUES ('87654321'); 

Note that only 1 field is inserted! Then, I SELECT from that table:

SELECT * FROM the_code_1; 

Result:

code the_code_location _________________________ 12345678 3 87654321 6 

Which is the 3rd character in both of those fields!

The table definition above has VIRTUAL, meaning that it's calculated on the fly. The fiddle also shows the use of the PERSISTENT keyword (in place of VIRTUAL), meaning that the data is physically stored on disk.

This is a fiddle using MySQL 5.7 - the only difference is that in place of the the PERSISTENT keyword, STORED is required. The MySQL syntax is here. This is also interesting:

col_name data_type [GENERATED ALWAYS] AS (expr) [VIRTUAL | STORED] [NOT NULL | NULL] -- Note! no PERSISTENT keyword! [UNIQUE [KEY]] [[PRIMARY] KEY] [COMMENT 'string'] 

All in all, depending on your disk versus performance requirements, one of these two options is the way to go - you can do a few tests. Your requirements are minimal so either of the two options should work satisfactorily.