MySQL stored procedure cursor not working - mysql

Question

MySQL 5.5
CREATE TABLE `card` (
`id` int,
`cardnumber` varchar(100),
`customer` varchar(100),
PRIMARY KEY (`id`)
);
INSERT INTO `card` VALUES (1, '5000', 'Google');
DELIMITER //
CREATE PROCEDURE `test` ()
BEGIN
DECLARE finished INTEGER DEFAULT 0;
DECLARE cardnumber varchar(20) DEFAULT "";
DECLARE cursor1 CURSOR FOR SELECT cardnumber FROM card WHERE customer = 'Google';
DECLARE CONTINUE HANDLER FOR NOT FOUND SET finished = 1;
OPEN cursor1;
get_card: LOOP
FETCH cursor1 INTO cardnumber;
IF finished = 1 THEN
LEAVE get_card;
END IF;
END LOOP get_card;
SELECT cardnumber;
CLOSE cursor1;
END //
DELIMITER ;
CALL test();
Returns no results, what did I do wrong?

Answer 1

There would be a ambiguity in identifying a column and a local variable if they are defined a same name. It IS always better practice to define a different naming convention in stored procedures, functions and trigger bodies when dealing with database object names like column.
To cross check, execute following procedure and see the result.
delimiter //
drop procedure if exists `same_name_test` //
create procedure `same_name_test`()
begin
DECLARE cardnumber varchar(20) DEFAULT "";
-- this statement will print empty string
SELECT cardnumber as cn
FROM card
WHERE customer = 'Google';
end;
//
delimiter ;
call `same_name_test`;
+------+
| cn |
+------+
| |
+------+
Modified Stored Procedure:
DELIMITER //
drop procedure if exists `test` //
CREATE PROCEDURE `test`()
BEGIN
DECLARE finished INTEGER DEFAULT 0;
DECLARE card_number varchar(20) DEFAULT "";
DECLARE cursor1 CURSOR FOR
SELECT cardnumber
FROM card_so_q23811277
WHERE customer = 'Google';
DECLARE CONTINUE HANDLER FOR NOT FOUND SET finished = 1;
OPEN cursor1;
get_card: LOOP
-- read cursor into new local variable
FETCH cursor1 INTO card_number;
IF finished = 1 THEN
LEAVE get_card;
END IF;
END LOOP get_card;
-- using new name of variable
SELECT card_number;
CLOSE cursor1;
END;
//
DELIMITER ;
Now call the Stored Procedure:
CALL `test`;
+-------------+
| card_number |
+-------------+
| 5000 |
+-------------+