MySQL构建变量名称？

Question

I am trying to build a variable name dynamically within a user defined function but it seems it does not work. Is there a way to do this or by using an array variable?

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

[Err] 1193 - Unknown system variable 'CONCAT'

Any hints on how I can achieve that? 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

You cant SET CONCAT as in SET CONCAT('D',x) = MID(days, x, 1) what instead i think will be better to declare & set the concat value to that variable and use that variable for the IF condition. Also I think you missed out what have to be done in the case IF Condition is true.

Answer 2

Thanks for your reply. I got it to work like this, but I am still curious, how I could dynamically create a variable in some other cases.

For this function, I realized I don't even need the variable, as I can evaluate it straight in the if clause.

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 ;