store function into variable that expects parameters

So given the following example query

SET @first_day_of_current_year = MAKEDATE(YEAR(CURRENT_DATE), 1);
SET @current_month = MONTH(CURRENT_DATE);

SELECT *
FROM document
WHERE
    created >= (SELECT DATE_ADD(@first_day_of_current_year, INTERVAL @current_month - 2 MONTH))
AND 
    created < (SELECT DATE_ADD(@first_day_of_current_year, INTERVAL @current_month - 1 MONTH));

I want to fetch all documents created in a range of two dates. As you can see I put some calculations into variables because I can reuse them in the WHERE clause. Further those calculations only differ by one value, the month at the end. So the calculation could also be

SELECT DATE_ADD(@first_day_of_current_year, INTERVAL @current_month - x MONTH)

Is there a way I could create a temporary function for that calculation that expects a parameter? Like so (pseudo syntax)

SET @calculator = amountOfMonths => SELECT DATE_ADD(@first_day_of_current_year, INTERVAL @current_month - {{amountOfMonths}} MONTH);

This would not improve the performance but it might improve the readability because things get reused.

Answer

You could use stored procedures to accomplish what you are looking for. Then you can pass it a parameter that you set as an OUT parameter, and use it after the procedure has been called.

Something like this;

DROP PROCEDURE IF EXISTS fetch_documents;
CREATE PROCEDURE fetch_documents(OUT foo integer)

BEGIN
    --here you set your out parameter equal to the calculation
end;

To call you can do something like;

CALL fetch_documents(@foo);

SELECT @foo;

Hope this helps!