MySQL prepared statement in a stored procedure not behaving as expected - mysql

Question

I have this MySQL Stored Procedure
DELIMITER $$
CREATE DEFINER=`dbuser`#`%` PROCEDURE `getTranslatedAnswer`(IN questionDesc VARCHAR(2500), col VARCHAR(10))
BEGIN
SET #paramVal = questionDesc;
SET #str = CONCAT('SELECT C.',col,' AS `answer`, D.',col,' AS `message`
FROM `option_group` A
INNER JOIN `questions_answers` B ON A.`option_id` = B.`option_id`
INNER JOIN `answers` C ON B.`answers_id` = C.`answers_id`
INNER JOIN `chat_message` D ON A.`option_group_id` = D.`option_group_id`
WHERE UPPER(`A`.',col,') = ? LIMIT 1');
PREPARE stmt FROM #str;
EXECUTE stmt USING #paramVal;
DEALLOCATE PREPARE stmt;
END
The above code when called using
call getTranslatedAnswer('我可以在机场买到预付SIM卡吗？','zh_chs');
will return 0 rows. Compared to as when I call the bare SQL below
SELECT C.zh_chs AS `answer`, D.zh_chs AS `message`
FROM `option_group` A
INNER JOIN `questions_answers` B ON A.`option_id` = B.`option_id`
INNER JOIN `answers` C ON B.`answers_id` = C.`answers_id`
INNER JOIN `chat_message` D ON A.`option_group_id` = D.`option_group_id`
WHERE UPPER(`A`.`zh_chs`) = '我可以在机场买到预付SIM卡吗？' LIMIT 1;
it returns 1 row. Am I missing something here? How do I debug this? Please advise, thanks!

Answer 1

I could be remembering wrong, but I seem to recall execute not actually returning results as you expecting it to.
What happens if you try something like this?
DELIMITER $$
CREATE DEFINER=`dbuser`#`%` PROCEDURE `getTranslatedAnswer`(IN questionDesc VARCHAR(2500), col VARCHAR(10))
BEGIN
SET #paramVal = questionDesc;
DROP TEMPORARY TABLE IF EXISTS `t_getTranslatedAnswer`;
SET #str = CONCAT('
CREATE TEMPORARY TABLE `t_getTranslatedAnswer`
SELECT C.',col,' AS `answer`, D.',col,' AS `message`
FROM `option_group` A
INNER JOIN `questions_answers` B ON A.`option_id` = B.`option_id`
INNER JOIN `answers` C ON B.`answers_id` = C.`answers_id`
INNER JOIN `chat_message` D ON A.`option_group_id` = D.`option_group_id`
WHERE UPPER(`A`.',col,') = ? LIMIT 1');
PREPARE stmt FROM #str;
EXECUTE stmt USING #paramVal;
DEALLOCATE PREPARE stmt;
SELECT * FROM `t_getTranslatedAnswer`;
DROP TEMPORARY TABLE `t_getTranslatedAnswer`;
END