Does MySQL build a variable name?

Question

I am trying to build a dynamic variable name in a user-defined function, but it seems that it does not work. Is there a way to do this or using an array variable?

I have a 7 character string that represents the days of the week (1234567 or 1_3_5_7, etc.). I would like to evaluate how often a day is selected during the week (from 0 to 7). I thought it would be easier to use a loop to go through all 7 positions, but I get an error

[Err] 1193 - Unknown system variable 'CONCAT'

Any hints on how I can achieve this? This is my code:

DELIMITER $$

DROP FUNCTION IF EXISTS fn_freq$$

CREATE FUNCTION fn_freq(days INT) RETURNS INT

BEGIN
        DECLARE D1 VARCHAR(1);
        DECLARE D2 VARCHAR(1);
        DECLARE D3 VARCHAR(1);
        DECLARE D4 VARCHAR(1);
        DECLARE D5 VARCHAR(1);
        DECLARE D6 VARCHAR(1);
        DECLARE D7 VARCHAR(1);

        DECLARE x INT;
        DECLARE fn_freq INT;
        SET x =1;
        SET fn_freq = 0;

        WHILE x < 8 DO
            SET CONCAT('D',x) = MID(days, x, 1);
            IF CONCAT('D',x) = '_' THEN
            ELSE
               SET fn_freq = fn_freq + 1;
               SET x = x + 1;
            END IF;
            SET x = x + 1;
        END WHILE;

        RETURN fn_freq;
END$$

DELIMITER ;

Answer 1

SET CONCAT as in SET CONCAT('D',x) = MID(days, x, 1), , concat IF. , , , , .

Answer 2

. , , .

, , if.

DELIMITER $$

DROP FUNCTION IF EXISTS fn_freq$$
CREATE FUNCTION fn_freq(days VarChar(7)) RETURNS INT
BEGIN
        DECLARE x INT;
        DECLARE fn_freq INT;
        SET x =1, fn_freq = 0;
        WHILE x < 8 DO
            IF MID(days, x, 1) <> '_' THEN
               SET fn_freq = fn_freq + 1;
            END IF;
            SET x = x + 1;
        END WHILE;
    RETURN fn_freq;
END$$
DELIMITER ;