MySQL Stored Procedure LIKE syntaxt in Cursor Declaration

Question

I am very green when it comes to MySQL Stored Procedures and i have been having trouble with selecting all payments received in the current month and looping through them to see which accounts did not pay the monthly fee, my problem seems to be with the LIKE statement in the declaration of the cursor. Here is the code:

CREATE DEFINER=`wisper`@`%` PROCEDURE `process_rejections`()
BEGIN
    DECLARE cursor_ID INT(11);
    DECLARE account_id INT(11);
    DECLARE amount_paid DECIMAL(10,2);
    DECLARE date_paid DATETIME;
    DECLARE cur_year INT DEFAULT (SELECT YEAR(CURRENT_DATE()));
    DECLARE cur_month INT DEFAULT (SELECT MONTH(CURRENT_DATE()));
    DECLARE comp_date VARCHAR(10) DEFAULT (SELECT CONCAT(cur_year,'-',cur_month));
    DECLARE done INT DEFAULT FALSE;

    DECLARE cursor_i CURSOR FOR 
    SELECT `id`, `account_id`, `amount_paid`, `date` 
    FROM `payments_received` 
    WHERE `date`LIKE CONCAT('%',@comp_date,'%');

    DECLARE CONTINUE HANDLER FOR NOT FOUND SET done = TRUE;
    OPEN cursor_i;
    read_loop: LOOP
        FETCH cursor_i INTO cursor_ID, account_id, amount_paid, date_paid;
            IF done THEN
                LEAVE read_loop;
            END IF;
            //Do Stuff here like change the status of the account
        END LOOP;
    CLOSE cursor_i;
END

IF i remove this:

WHERE `date` LIKE CONCAT('%',@comp_date,'%');

Everything works but it obviously selects * payments received instead of those that occured during ,for example, Aug 2017. I do not want to select * because over time there will be hundreds of thousands of rows in the specific table and i do not want the overhead to become too much. I have also tested this:

WHERE `date` LIKE '2017-08-11';

Which also does not work. During debuggin i also tried to use a specifi date like this:

WHERE `date` = '2017-08-11';

And everything worked nicely but it is obviously just to test and see if the WHERE cluase was syntactically correct inside the cursor declaration.

I am at a lost here and would appreciate some assistance from you guy's.

Thank You in advance.

Answer 1

You've debugged your code well, and you've isolated the problem successfully. It's not the LIKE itself, or the fact it is a stored procedure that's tripping you up.

Problem : You're using a session variable (the variable is prepended with an @ ), not the stored procedure variable declared a few lines earlier.

Solution : Drop the @ (on comp_date ):

WHERE `date` LIKE CONCAT('%', comp_date, '%');

Assuming the session variable isn't set, I believe you'll be getting NULL , which won't work. Thus the symptom you're seeing.

There's some useful explanation on another SO question here: MySQL: @variable vs. variable. Whats the difference?

Answer 2

I have found the problem, turns out the comp_date looks like this 2017-8 instead of using leading zeros like this 2017-08, here is the finished code:

PS: Yes i feel like an idiot for not testing the values that i submit in the statement, but maybe someone else also struggles with it and this can help them.

CREATE DEFINER=`wesley`@`%` PROCEDURE `process_rejections`()
BEGIN
    DECLARE cursor_ID INT(11);
    DECLARE account_id INT(11);
    DECLARE amount_paid DECIMAL(10,2);
    DECLARE date_paid DATETIME;

    DECLARE comp_date VARCHAR(10) DEFAULT 
    CONCAT(YEAR(CURRENT_DATE()), '-', LPAD(MONTH(CURRENT_DATE()), 2, '0'));

    DECLARE done INT DEFAULT FALSE;
    DECLARE cursor_i CURSOR 
    FOR SELECT `id`, `account_id`, `amount_paid`, `date` 
    FROM `payments_received` 
    WHERE `date` 
    LIKE CONCAT('%', comp_date,'%');

    DECLARE CONTINUE HANDLER FOR NOT FOUND SET done = TRUE;

    OPEN cursor_i;
    read_loop: LOOP
    FETCH cursor_i INTO cursor_ID, account_id, amount_paid, date_paid;
            IF done THEN
                LEAVE read_loop;
            END IF;
        //Do stuff here like changing account status
        END LOOP;
    CLOSE cursor_i;
END

Note how i declare comp_date to account for leading zeros.